Alright, I recently got exo-force books 1-5 again, and even though I've read through them before, I wanted to make sure that my favorite was still the same, and it is.
Book 1 is probably my second favorite, its got enough action in it to make it fun, and it has a point to it. To rescue Takeshi's family. Though my main problem with this book is its far too short and only centers around one character, Hikaru. If you don't mind a slow-going start and like adventure more then action, I would recommend reading this book.
Books 2 and 3 I won't even go into detail on, they are just slightly better worded versions of comics 18-28, and they don't cover any new events or anything that wasn't in the comics. Due to these essentially being the same as the comics, I wouldn't recommend even reading these.
Book 4 is my favorite, with out a doubt. The only of the five books that is completely its own, featuring nothing from the original comics, it has a perfect mix of action, adventure, and even throws in a slight tint of horror that you don't see often in Exo-Force. Its storyline is great, and it kept me on the edge of my seat even reading through it my second time, and I know how it ends. I would highly recommend reading this book.
Book 5 is...ok. It wouldn't be so bad if it didn't have so much conflicting information, unlike the previous four books, its written by Allison Lassieur, rather then Greg Farshtey, and I think that's why it has so many errors. To begin with, it conflicts with the comics by having Hitomi steal the blazing falcon, but she already had done that in the comics, and she did it to save Ha-Ya-To, not to go in search of a code. My biggest nitpick is with the ending part, when we see three mobile devastators attack the Golden City. There was only one, there will only be one, and there will never be more. The book has some good pictures that weren't in previous books or comics (for the most part), but it still has a lot of errors. If you can overlook the conflicting canon information, I'd say give this book a shot.