http://books.google.co.uk/books?id=Gtv62NnHrOcC&printsec=frontcover&dq=Casual+Game+Design:Designing+Play+For+The+Gamer+In+All+Of+Us&hl=en&ei=OuDETufsF4ik8gPpr-GCCw&sa=X&oi=book_result&ct=result&resnum=1&sqi=2&ved=0CEEQ6AEwAA#v=onepage&q&f=false

Page 41 talks about level design, I hadn't seen this before but thought it best to save the link for use later.