Stephen Cave's playful approach to flatworms suggests his forthcoming book will be a good read. Let's hear it for monkey testicle grafts.
There's a chapter on the flatworms (and other worms) in The Book of Barely Imagined Beings. I signed off final corrections in January but the book doesn't appear until October.
P.S. A striking image of mating terrestrial flatworms
P.P.S. A Cave tweet