The first and third book are written by
Marc Cerasini, who has written a lot of television and movie adaptations. I've read his first book and it was good, but it felt light-weight to me.
The second and so-to-be-fourth book are written by John Whitman. In addition to the "24" novels, Whitman wrote for the software series Where in the World is Carmen Sandiego? and wrote the novel series Star Wars: Galaxy of Fear. I haven't finished the second book but it has a very different tone and feel than the first one. Oddly enough for a young adult writer, his book is more solid and feels denser than the first one. Jack isn't quite as invincible (so far) in this either, but you get a lot more introspection from him in it. I'll have to finish reading it to see how it rates.
As for control over the novels, that depends entirely on the deal the publisher made with Fox. Usually the books have to stay within the established "24" universe (and so far they do) and usually the creators and Fox have a say in the plot. The writers are usually free to add their own thoughts to the characters. Some shows -- like Star Trek -- have books that completely blow canon away, others -- like the Stargate -- don't do anything to negate what the show has established.