Be careful, when building the U pentomino, to be certain to use a spacer die in the hollow of the U when you glue. Otherwise you may end up with a stubborn U that will accept another block only reluctantly.
A classic book on polyomino puzzles of all types is Solomon W. Golomb's Polyominoes published in 1965. It's interesting reading, if you want an in-depth study of the problem and a gaggle of interesting problems, but it's by no means necessary. There's more than enough polyomino information on the web to satisfy all but the most ardent curiosity.