By which author was your last read? Let's pick the mentioned Terry Pratchett.
Then go here:
http://www.literature-map.com/
Type in "Terry Pratchett" without the quotes. The cloud that appears, shows authors that write in a similar style to Pratchett. Pick one of the authors on the outer edge, so you know it is somewhat similar to Pratchett, but not too much.
Find a book (if it's in a series, obviously the first one) by this author and read it.
I find music in a similar way, TuneGlue:
http://audiomap.tuneglue.net/
Type in your singer/band, and then expand it to find similar ones.