The books for the most part feature one of three teenaged heroes: Johnny Dixon (hero of the Curse of the Blue Figurine and many others), Anthony Monday (first appearing in The Treasure of Alpheus Winterborn), and Lewis Barnavelt (who first appeared in the first of Bellairs' young adult books, "The House with the Clock in its Walls"). Each of the respective heroes has their share of adult mentors and child peers who play roles that grow and diminish from book to book.
The subject matter of the books often includes references to the occult and to religious imagery. There are usually supernatural powers at play and some pretty scary stuff tends to happen (in my opinion).
Sadly, I learned while researching for this entry that John Bellairs passed away in 1991, but another author, Brad Strickland, was hired to complete two unfinished manuscripts and to write novels based on some brief descriptions Bellairs left behind. Since completing those Strickland has been authorized to write books with the same characters and has continued to do so with the latest book being published this year. I haven't read any of the Strickland penned books, and on the surface, I hate the idea of someone writing books about characters they didn't invent. Feels too much like fan fiction. But as I haven't read any of them, I hesitate to decry them at this juncture.
A few of the books that stand out most to me from the collection are:

The Curse of the Blue Figurine

The House with the Clock in its Walls

The Treasure of Alpheus Winterborn

The Revenge of the Wizard's Ghost

The Eyes of the Killer Robot
That said, I think I enjoyed pretty much all of Bellairs' works and highly recommend them for young adults and adults who don't mind reading a book written for young adults (Harry Potter, anyone?).
Good References:
Wikipedia: John Bellairs
Bellairsia