I've found both books to be very helpful!
The Ethical Slut books are the first edition (1998) and the 2nd, revised and expanded edition (2009) respectively. Either are fine - if you can afford it, maybe the later version is better but I doubt the material has changed all that much.
As for Sex at Dawn, this is the same book, one seems to be the audiobook version and the other seems to be the upcoming paperback edition. Again, same book, different packaging. Again any version is fine.
I've had good luck getting "Sex at Dawn" at the library. Might be worth a try. (Ethical Slut - not so much!)