A lot of scholarly literary people do not really like series books. There seems to be a feeling that they aren't quality literature and should therefore be avoided. Then, within series there are the "good" kind and the "bad kind." The good series have books all written by the same author and a new book comes out only once a year or less (ie. Harry Potter, Ramona by Beverly Cleary, etc). Bad series are books that are written by multiple authors and published more than once a year (Nancy Drew, Baby-sitters club, etc).
I loved series books while I was growing up, especially the "bad" ones. I think I read every single Nancy Drew Casefile book published until I was a sophomore in high school. It drove my mother crazy, but they were fun and easy and there is just something so relaxing about "knowing" the story before you even start.
One of my current favorite "bad" series is the Disney Fairies series. The stories follow the adventures of Tinkerbell and her other fairy friends in Neverland. Peter Pan only appears in the books where Tink is the main character, otherwise all the characters are original.
While the books aren't going to become classics in the every child should read them sense, the stories are simple and usually have one direct positive message (being nice to friends, allowing yourself to ask for help, etc). They are written for a second grade reading level and definitely aimed at girls. And, if they get someone to read, that's more important than the fact that they aren't reading a classic. Isn't it?
What I really love about the Disney Fairy books is the beautiful color pictures that are in each of the stories. These are not cheap black and white (I do know that all black and white pictures are cheap), these are full color, often full page, beautiful drawings of scenes in the book. The covers can kind of give you an idea, but I think most of the best pictures are actually within the pages of the book.
And, just for the record, currently my favorite fairies are Tink (of course) and Fira.