Depends on where you are in your base knowledge. Supertraining and Essentials are great if you have a decent foundation. If not Siff can take some real swimming to get through. I went to the bookstore a few weeks back trying to find easier read books for beginners to get them some sort of foundation outside of all the garbage they might get in the local gym or magazines. I found 3 books that any store should have that are a wealth of information in a very digestable format.
I train police cadets and recruits, and their varying levels of experience and background are considerable. Most police training programs I know of are Cooper based and other old inefficient modalities of training…what I told them to get if they are interested more in understanding the programs I am putting them through are:
Core Performance, Mark Verstegen
New Rules of Lifting, Schuler and Cosgrove (Ive only read the first 50 pages or so and skimmed the rest, I am really liking this book as far as explaining things in laymens terms to people who might have believed bullcrap hype from idiots their entire lives. I wish I could afford a few hundred of these books to pass out to new recruits and instructors which should be mandatory read)
Muscle Logic, Charles Staley (lots of good pictures, of course its promoting his EDT theory but still I think its a great book for someone to read that is just starting off, with lots of good pictures)
If you are more advanced, go with Siff and Essentials, sign up to the NSCA to get the Journal, sign up for Milo and get those (not always technical articles but very motivating to a meathead like me), and Im not sure if they are still in print but back in the day Hatfields Power and Hardcore Bodybuilding were great although I havent looked at them in years. If you want to see examples of more than just lifting, warm ups, speed and agility stuff, Complete Conditioning for Football by Arthur/Bailey is good, Renegade Trainnig for Football by Davies has good pics of dynamic flex/hurdles/ladder/lifts.
Good videos by Verstegen, Boyle, Santanos, Parisi Speed School, are other educational sources.