What’s Next?
A great site for readers to find information concerning adult fiction in series (two or more books linked by character, setting, etc.). Search by author, title or series to get the proper sequence of titles. The What’s Next™: Books in Series database was developed and is maintained by the Kent District (MI) Library

Juvenile Series and Sequels
The ‘Juvenile Series and Sequels’ database contains over 36,000 books in 4,900 series titles that are classified into three audiences: Juvenile Easy [JE] – for birth through 2nd grade readers; Juvenile [J] – for 2nd through 6th grade readers; Young Adult [YA] – for 6th through 12th grade readers. From the Mid-Continent (MO) Public Library.

LibraryLists’ Series and Sequels
A great collection of links.