Ok since you don't say if you want the same type of book ie a hero series nor an age
If they want more magic
A Wizard in Rhyme is a series by Christopher Stasheff.
Myth Adventures series Robert Asprin - warning lots of PUNS
Xanth by Peirs Anthony - warning lots of PUNS
Sci Fi and Fantasy
The well world series and The Dancing Gods series by Jack Chalker
The Stainless Steel Rat by Harry Harrison - a good young reader book series
The Pern series by Anne McCaffrey
I have given many of the well world books away as gifts to older teen readers, the other books are fine for early teens and pre-teens as well as adults. The Xanth series is a LOT of fun to read.