With The Ethical Slut, the 2nd edition would probably be best. I think it contains most, if not all, of what was in the first edition; mostly just editions. I don't know for sure though, I've only read the first edition. Although, I keep meaning to order a copy of the second.
When I read it, I thought "I've spent the past few years figuring most of this out on my own, and there's been a book this whole time?!" However, there were some things in it I hadn't considered, and it was definitely worth reading. I'd recommend it.
Sex at Dawn I haven't read.