Almost all the books by these folks have demanding material:
Daniel Ho
Lyle Ritz
Tony Mizen
John King
Fred Sokolow
Rob MacKillop
Mark Nelson
(And I'm sure there are more)
But how about thinking outside the box a little? There is a ton of material written for the ukulele's close relatives, the renaissance guitar, the machete, and the cavaquinho, that transfers perfectly to the uke.
Then think way outside the box and play material that
A. was written for other instruments (e.g. plenty of virtuosic material was written for recorders that fits well on the uke.)
B. was written for any instrument (e.g. Jazz improvisation)
The above suggestions should keep you busy for at least a few lifetimes!
p.s. I left out Jake Shimabukuru because I figured you only wanted material that was humanly possible! : )