I think "Seize the Night" has a great many details about the character Acheron, who has a feature book coming in August. So it is a good book to review details about him. Yes, you really ought to read the whole series, but you can read one at a time. I'm now RE-reading the series, in preparation for the August book.
P.S. THANKS for continuing to post about free books, when you find them. Some of us seem not to ever hear about them, otherwise.