Book vs. Movie (SPOILERS)
I just got the book a few days ago and have zoomed through most of it, and I will say the book is very readable in its own right and also significantly different in many ways from the movie.
A few major differences:
1) The time frame. In the movie, the storyline is apparently spread over one school year, September (Homer getting his butt kicked in football), to May-June (launch of final rocket). In the book, the storyline stretched for about three full years (fall 1957-spring 1960).
2) The amount of help Homer/Sonny gets from his dad. In the movie Dad does nothing to help Homer until the very last night of the national science contest; in the book he does quite a lot to help him (including diverting some company resources).
3) In the book Sonny does not quit school to support the family after Dad's injury (in the book he only misses a week or so of work)
4) As others have pointed out, the number of "Rocket Boys" -- in the book there are six kids involved in the rocket building.
5) In the book many more details are given on Sonny/Homer's romantic exploits with the opposite sex.
6) In the book the rivalry between Homer/Sonny and Jim is much more nasty and bitter. Jim basically acts like a complete tool for the whole book.
7) As an outgrowth of that, "the football boys" are depicted throughout the book as villains. Although there are a couple scenes in the movie where the football boys pick on the rocket boys, in the book it is just constant, like the football boys were sitting around plotting how to mess with the rocket boys.
As a partial member of both cliques in high school I can attest that at the vast vast majority of all high schools, the "football boys" rarely spend more than 30 seconds thinking about the "rocket boys", at all.
There are many many other differences, some minor, some major, between the book and movie.
One amusing difference is Quentin: in the book, he was depicted as actually being fairly good-looking (as opposed to totally dorky in the movie), although he was still considered an egghead and was outcast from most of the kids.
Quentin in the book also constantly speaks in an affected "British" accent, using absurdly flowery condescending scientific jargon and being very snotty to pretty much everybody he meets, including his best friends. Easy to see how a kid like that would be left to eat his lunch by himself -- not to mention catching a knuckle sandwich or two.