SERIES AND SEQUELS: Not sure which books is next is series or how many books there are in total, Mid-Continent Public Library has a compiled over 4,900 series which accounts for over 36,000 books.
Mid-Continent Public Library via Missouri
This tool has eleven different categories (e.g. happy/sad, funny/serious, optimistic/bleak, etc...). The user indicates their preference via sliders in these various categories in order to get recommendations.
Put the title of a book you've enjoyed in the past and this handy resource can provide recommendations for up to 8 other books you make also like.
Similar to yournextread. You enter in a book or author you enjoy and this site will recommend similar titles you may enjoy. I would recommend using yournextread first.