Slugs and Snails (Minibeasts) - Claire Llewellyn
Can't say I'd pondered it much before, but now that I have the visual image in my head it's probably there to stay. I'm talking about slugs mating. Yes, this book goes there - which I, admittedly, found kind of awesome and horrifying at the same time. I think this book contains just about everything I personally ever need to know about snails and slugs - i.e. it's quite thorough. Good photos (sometimes too good) and nice drawings as well.