While there is plenty of great content that you should try to read at some point in the first two books, it's hard to not recommend the Forever book, it's just so damn complete and straightforward. I almost hope he never releases another as there's just so much stuff I want to try from it, and it seems like it's going to take at least a decade.
If you have a significant grasp of the following I think you may be able to go straight to 5/3/1 Forever:
-What 5's progression is
-setting TM, adjusting TM, for the love of god not increasing it too much for a good PR, etc
-Spinal tap, Beyond programming nice to know but only necessary for a few programs in Forever (so get the Forever book now, and Beyond later maybe?)
-The ideas, philosophy around how the original program came to be, kicking ass with just the triumvirate, etc
-Not adding tons of dumb shit and "training like an asshole" as the new book likes to say
So if you've done your homework (i.e. the numerous articles here, Jim's site, old forum posts while focusing mostly on JIM'S responses), tried the program for a while, you might be ok. If you skimmed the article and relate to some of the more basic questions that show up on here (no offense), maybe best to just pick up the original one first. You could still get plenty of mileage out of it.