rec.puzzles.crossword - via Google
At both these places you can enter clue setting competitions.
People will queue up to criticise your efforts, for free.
'Ximenes on the Art of the Crossword'
Yes, an actual paper-and-ink book.
Rather old, but recently reprinted and considered
to be 'the tablets of stone' for setting crosswords
'Chambers Crossword Manual'
another one. ISBN 0-550-12006-8