Several years ago I picked up the first Fablehaven book at random from the library. The first half was okay, but near the end I found myself getting genuinely freaked out. Not that it was violent or disturbing, Brandon just did a great job making the story seem real. I decided to give the second book in the series a try and I'm glad I did. (It was much better than the first.) Thus began my like-alot-affair with Mr. Mull. I've read most of his books and there's nary a swear word to be found. Nor is there anything sexual. The only thing parents might need to watch out for is mild fantasy violence and kids disobeying their elders. The covers below each represent entire series, so if you find you like Brandon's style you'll have enough material to last you through adolescence. (Or your midlife crisis.)
SURGEON GENERAL RATING - GIn general, the Fablehaven books get better as they go. (Although #2 will always be my favorite.) The Candy Shop Wars I enjoyed least, and I still haven't read the sequel. The first and second books in the Beyonders series were phenomenal (if you can overlook that stupid hippo) although by the third book, Brandon's horrible prose and meandering plotting was starting to get to me. The book still sits half finished in my locker at work.
I reiterate, the writing in these books is sub-par. Don't expect to be intellectually stimulated. Brandon does a whole lot of telling instead of showing and he loves him some adverbs. But if you're not a lit-snob like me, you shouldn't have any trouble losing yourself in the incredibly creative stories.