The Boxcar Children books are some of my favorites. If you aren't familiar with this series, it is about four children ages six to fourteen who solve mysteries. With over 130 books, I still haven't read them all! Of course, there are some that I haven't read simply because their titles don't interest me, such as The Mystery of the Runaway Ghost. In some books, the characters think that a location is haunted, but it always turns out that the "ghost" is a prankster.
If you want to read adventure/mystery books, but don't like boatloads of suspense, these are the books for you. I would recommend these books for ages seven and up, and give them a rating of four and a half stars. If you want to find some Boxcar Children books, most libraries have them.