The Post-Tensioning Institute would be your best bet for finding useful resources on the subject. I have a copy of their manual for design of post-tensioned slabs-on-ground, and they also publish a variety of other useful literature, including the Post-Tensioning Manual. Their website is located at
The sixth edition of the post-tension manual came out last year. It is probably one of your best bets. As far as concepts one of the best books out there is Collins and Mitchell Prestressed Concrete