Not a topic about a specific book but rather your format of choice.
I find a nice balance between all of them.
First, you can't beat a good hard copy. I usually buy mine second hand and don't feel bad about writing in the margins and highlighting specific sections. I know hard back is the collectors choice but the majority of mine are paper back. Maybe one day I'll own a nice leather bound set of my favorites...you know, after I hit the lotto.
I will forever preach the benefit of e-books. You simply cannot beat the search feature they offer. It's also nice to have a tablet or iPad with you on the field instead of a backpack of books. I'm blessed to have most of my Gettysburg reference books as e-books as well as hard copies.
Audiobooks are my favorite. It seems my retention for information is best when I hear it. Yes. I have also read an e-book while listening to the same book on audio, following along with the reader.
As anybody who enjoys audiobooks as well as I do can tell you, it depends on the reader. I've listened to some ( Stephen Sears Gettysburg to be exact) that made me want to gouge my ear drums out with a red hot poker. Grover Gardner is my favorite (the guy who reads Shelby Foote's three volume classic on the Civil War).
Just curious as to where you gentlemens affections lie when it comes to "books".