This situation is recognised by the APA referencing rules. If there are no page numbers, but paragraph numbers are available, use the paragraph and indicate this by using the abbreviation 'para.'.
For example "your quote" (para.4). If there are no page or paragraph numbers, but your quote is in a section of the book with a heading, count the number of paragraphs yourself and use that as the paragraph number.
For example "your quote" (Discussion section, para. 5). If the section heading is rather long, you can shorten it at a sensible point, but you need to present it in quotation marks eg "your quote" ("Discussion section", para.5.).
There is more detailed guidance on the APA system via the University Library's Website.
