I estimate that generally the latest edition of a book is preferred unless you're after collection value or if an older version is preferred for some reason, for example due to original cover. In non-fiction new or updated chapters may be added. Translated fiction literature perhaps sees the most revisions. But even if the new edition has only been expanded by the author, the differences are still valuable information to be had and can be used to compare the book to its release period.
Your question "How can I find information about the editions of a book?" has no simple general answer. There is a whole field of learning devoted to it: "bibliography". Basic questions in that field are: what are the various editions of such and such a book, where and when did they appear, what are the relationships between them, what are the differences between them?
The answers here depend on the purpose of the questions. Often, for instance, publishers will print essentially the same book with identical contents but different title-and-copyright pages showing (say) London as the place of publication of the one edition and New York as the other. For some purposes I suppose this counts as one edition, for others, as two.
Depending on the era, and kind of book, and importance of the book, there might be shortcuts. Library catalogs often help a lot. For very recent books (after the invention of the ISBN, say) there are various databases used by the book-selling industry. You can ask experts in the subject matter of the book in question. For instsance, if I wanted to know about the various editions of the work of the cartoonist Charles M. Schulz, I'd ask a friend who is a scholarly Peanuts fiend.