Wikipedia:Citing sources
Page numbers are not required for a reference to the book or article as a whole. When you specify a page number, it is helpful to specify the version (date and edition for books) of the source because the layout, pagination, length, etc. can change between editions.
If there are no page numbers, whether in ebooks or print materials, then you can use other means of identifying the relevant section of a lengthy work, such as the chapter number or the section title.
In some works, such as plays and ancient works, there are standard methods of referring to sections, such as "Act 1, scene 2" for plays and Bekker numbers for Aristotle's works. Use these methods whenever appropriate.

Audio and video sources: Specify the time at which the event or other point of interest occurs. Be as precise as possible about the version of the source that you are citing; for example, movies are often released in different editions or "cuts". Due to variations between formats and playback equipment, precision may not be accurate in some cases. However, many government agencies do not publish minutes and transcripts but do post video of official meetings online; generally the subcontractors who handle audio-visual are quite precise.

Links and ID numbers: A citation ideally includes a link or ID number to help editors locate the source. If you have a URL (web page) link, you can add it to the title part of the citation, so that when you add the citation to Wikipedia the URL becomes hidden and the title becomes clickable. To do this, enclose the URL and the title in square brackets—the URL first, then a space, then the title. For example:

For web-only sources with no publication date, the "Retrieved" date (or the date you accessed the web page) should be included, in case the web page changes in the future. For example: Retrieved 15 July 2011 or you can use the access-date parameter in the automatic Wikipedia:refToolbar 2.0 editing window feature.
You can also add an ID number to the end of a citation. The ID number might be an ISBN for a book, a DOI (digital object identifier) for an article or some e-books, or any of several ID numbers that are specific to particular article databases, such as a PMID number for articles on PubMed. It may be possible to format these so that they are automatically activated and become clickable when added to Wikipedia, for example by typing ISBN (or PMID) followed by a space and the ID number.
If your source is not available online, it should be available in reputable libraries, archives, or collections. If a citation without an external link is challenged as unavailable, any of the following is sufficient to show the material to be reasonably available (though not necessarily reliable): providing an ISBN or OCLC number; linking to an established Wikipedia article about the source (the work, its author, or its publisher); or directly quoting the material on the talk page, briefly and in context.

Linking to pages in PDF files: Links to long PDF documents can be made more convenient by taking readers to a specific page with the addition of #page=n to the document URL, where n is the page number. For example, using https://www.domain.com/document.pdf#page=5 as the citation URL displays page five of the document in any PDF viewer that supports this feature. If the viewer or browser does not support it, it will display the first page instead.

Linking to Google Books pages: Google Books sometimes allows numbered book pages to be linked to directly.
Page links should only be added when the book is available for preview; they will not work with snippet view. Keep in mind that availability varies by location. No editor is required to add page links, but if another editor adds them, they should not be removed without cause; see the October 2010 RfC for further information.
These can be added in several ways (with and without citation templates):

Rawls, John. A Theory of Justice. Harvard University Press, 1971, p. 18.
Or with a template: Rawls, John (1971). A Theory of Justice. Harvard University Press. p. 18.
Rawls 1971, p. 18.
Rawls 1971, p. 18.
Rawls 1971, p. 18.
Rawls 1971, 18.
In edit mode, the URL for p. 18 of A Theory of Justice can be entered like this using the {{Cite book}} template:

or like this, in the first of the above examples, formatted manually:

When the page number is a Roman numeral, commonly seen at the beginning of books, the URL looks like this for page xvii (Roman numeral 17) of the same book:
     https://books.google.com/books?id=kvpby7HtAe0C&pg=PR17
The &pg=PR17 indicates "page, Roman, 17", in contrast to the &pg=PA18, "page, Arabic, 18" the URL given earlier.
You can also link to a tipped-in page, such as an unnumbered page of images between two regular pages. (If the page contains an image that is protected by copyright, it will be replaced by a tiny notice saying "copyrighted image".) The URL for eleventh tipped-in page inserted after page 304 of The Selected Papers of Elizabeth Cady Stanton and Susan B. Anthony, looks like this:
     https://books.google.com/books?id=dBs4CO1DsF4C&pg=PA304-IA11
The &pg=PA304-IA11 can be interpreted as "page, Arabic, 304; inserted after: 11".
Note that some templates properly support links only in parameters specifically designed to hold URLs like |url= and |archive-url= and that placing links in other parameters may not link properly or will cause mangled COinS metadata output. However, the |page= and |pages= parameters of all Citation Style 1/Citation Style 2 citation templates, the family of {{sfn}}- and {{harv}}-style templates, as well as {{r}}, {{rp}} and {{ran}} are designed to be safe in this regard as well.
Citer may be helpful.
Users may also link the quotation on Google Books to individual titles, via a short permalink which ends with their related ISBN, OCLC or LCCN numerical code, e.g.:
https://books.google.com/books?vid=ISBN0521349931, a permalink to the Google book with the ISBN code 0521349931.
For further details, you may see How-to explanation on support.google.com.

Say where you read it: "Say where you read it" follows the practice in academic writing of citing sources directly only if you have read the source yourself. If your knowledge of the source is secondhand—that is, if you have read Jones (2010), who cited Smith (2009), and you want to use what Smith (2009) said—make clear that your knowledge of Smith is based on your reading of Jones.
When citing the source, write the following (this formatting is just an example):

John Smith (2009). Name of Book I Haven't Seen, Cambridge University Press, p. 99, cited in Paul Jones (2010). Name of Encyclopedia I Have Seen, Oxford University Press, p. 29.
Or if you are using short citations:

Smith (2009), p. 99, cited in Jones (2010), p. 29.
The same principle applies when indicating the source of images and other media files in an article.
Note: The advice to "say where you read it" does not mean that you have to give credit to any search engines, websites, libraries, library catalogs, archives, subscription services, bibliographies, or other sources that led you to Smith's book. If you have read a book or article yourself, that's all you have to cite. You do not have to specify how you obtained and read it.
So long as you are confident that you read a true and accurate copy, it does not matter whether you read the material using an online service like Google Books; using preview options at a bookseller's website like Amazon; through your library; via online paid databases of scanned publications, such as JSTOR; using reading machines; on an e-reader (except to the extent that this affects page numbering); or any other method.

Dates and reprints: Date a book that is identically reprinted or printed-on-demand to the first date in which the edition became available. For example, if an edition of a book was first released in 2005 with an identical reprinting in 2007, date it to 2005. If substantive changes were made in a reprint, sometimes marked on the verso with "Reprinted with corrections", note the edition and append the corrected reprint year to it (e.g. "1st ed. reprinted with corrections 2005").
Editors should be aware that older sources (especially those in the public domain) are sometimes republished with modern publication dates; treat these as new publications. When this occurs and the citation style being used requires it, cite both the new and original publication dates, e.g.:

Darwin, Charles (1964) [1859]. On the Origin of Species (facsimile of 1st ed.). Harvard University Press.
This is done automatically in the {{citation}} and {{cite book}} templates when you use the |orig-date= parameter.