There are 2 books written by A. Graham Bell from England and sold through Motorbooks International. One is on 2 strokes and the other on 4 strokes. However they are both a bit dated.
I produced videos on the subject that are basically an overview of performance techniques like what areas to pay attention to and what tools to use. If you're looking to do engine rebuilding, those videos are filmed in "slow-time" meaning you can follow along right in your own garage. The videos are listed on my site and you cxan click on my logo on the left column of this page.
Thanks, Eric