This series is based on a popular Disney cartoon, and reads much like any other "novelization" of a TV show or movie. The hook is the fact that what was once a cartoon is now a book; happily the actual story and the writing are worthwhile on their own.
That's especially fine if what you want is an angle to get a kid who likes the cartoon to start doing some reading. But if you are looking for a book that really stands out on its own by virtue of its story, or characters or the quality of the writing, well that's not entirely what Disney is trying to do here.
In all of the Phineas and Ferb books the plot is the same: Phineas and Ferb accomplish something difficult with ridiculous ease; this frustrates the sister Candace, (who I think seems like an OK sister and shouldn't always have to be the victim); meanwhile Perry the Platypus, their pet, who is really a secret agent, gets involved in some standard secret agenty stuff. That's pretty much it. Happily, there aren't any farts or boogers, which is good; and there is an occasional nod in the direction of sly or subversive humor, (which is welcome and not always the Disney style).
So, certainly a fine book to have in the family library, and one I might search for if my reader were really into the cartoon show.