While books purchased in the iBooks store should remian in your Purchased list, and be downloadable again for free; there is an exception. If the book was removed (by the publishser) from the iBooks Store.
If the book is only removed for sale, then the book will no longer show up in search results, but should still appear in the Purchased tab, and be downloadable.
If the book is completely removed, it may or may not still show in the Purchased tab, but either way, it can no longer be downloaded.
If the book still shows up in a search, does not have a dowload icon (cloud wiht down arrow), and you are absolutely sure you purchaced it; then you should contac Apple's iTunes support and see if they can restore your purchase. Hold in mind that the book may have been replaced by a newer eddition, and the publisher is under no obligation to make the new version availale for free.
Here is an Apple Support page that may help:
Download your past purchases - Apple Support