iBooks will only read books from the iBook Store, non-DRM ePub books, and non-DRM PDF files.
If you bought the book from another source, chances are it has DRM. Very few ebook retailers sell DRM free ebooks, at least not books still in copyright.
Go back to the site where you bought the book and look for any indication that they support the iPad, either with DRM free ePubs, or with their own app. Just because they sell a book about the iPad is no guarantee that they support it, though not doing so is kind of ironic.
The book "Talking to Siri" came out first on Amazon, which is also slightly ironic.
When I go to "Books" (in iTunes) on my computer, all the book covers are shown. Question : is it possible to make a folder/library within "Books" to keep all the titles from the same author together (to try and tidy up the "Books" library ?
There are several views you can browser iTunes with. Check the top bar for them. Not all will be available for all library types, but these four can be used in the Books library. Try each out and change they way they sort by selecting the column headers or other options that may appear at the top of the browser.
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If you want lists of specific authors, genre, or just about any category or combination of; go to the Files list and create a Smart List and play around. It's very powerful tool.
It is under the File menu: New Smart List.