Dead Tree format? Digital? Both? Take heed of my misfortune.
I have been cataloguging my book collection and updating my Library on here. I have physical books, iBooks from Apple, Kindle books from Amazon, book summaries on Blink (because I like to read books before I read them, if you know what I mean), and a large variety of old out-of-print books that exist only in PDF format.
I have found a second ebook that has disappeared from my iBook/Kindle collection. They are just not there, not in the "offline" section, just deleted and dropped into the Memory Hole. The first one I didn't mind, I got it to review it and it's the kind of book I won't pick up again. The second book was, and that angered me to a good degree.
Because of this, while I catalog all of my books, I am assessing the ebooks for a DTF (Deat Tree Format, paper) purchase. Don't get me wrong, having dozens, hundreds, thousands of books on a tablet is a great resource, and they don't weigh an ounce. However, if you are accessing them through an application like iBooks or Kindle, they might just disappear like Nineteen-Eighty Four did some fifteen years ago.
In the iBooks/Kindle, you're not purchasing a copy of the book like you do in DTF, you purchase a sub-license, contingent on the license that is held by Apple/Amazon. They lose their license, "your" copy goes *POOF*. Or a glitch happens in their database and they have no record you even purchasing it.
Lesson learned is, keep physical copies of books you want to reread or reference from time-to-time. Or, make them PDF's and load them into your phone/tablet into an app so that Amazon, Apple or whoever can't just yank them off your device.