Here are the details from publisher Ace Books:
A Hard Day's Knight
Simon R. Green - Author
Hardcover | 9.25 x 6.25in | 304 pages | ISBN 9780441019700 | 04 Jan 2011 | Ace | 18 - AND UP
John Taylor is a P.I. with a special talent for finding lost things in the dark and secret center of London known as the Nightside. He's also the reluctant owner of a very special-and dangerous-weapon. Excalibur, the legendary sword. To find out why he was chosen to wield it, John must consult the Last Defenders of Camelot, a group of knights who dwell in a place that some find more frightening than the Nightside.
London Proper. It's been years since John's been back-and there are good reasons for that.
Amazon.com has additional information from Publisher's Weekly. Apparently the series has always had some Arthurian content, but this seems the most Arthurian (I think).
Private investigator John Taylor returns in the 12th novel (after 2010's The Good, the Bad, and the Uncanny) of the Nightside, "the dark, secret, brooding heart of London." Having killed the former Walker, the voice of the Authorities, Taylor's now the closet thing the Nightside has to a leader. When the sword Excalibur shows up in the mail, he must return to London Proper to consult with the descendants of the Knights of the Round Table. The blood runs red (and golden) as he teams up with King Arthur and tangles with Merlin, elven royalty, traitor knights, sons of the devil, the Lady of the Lake, and various creatively named denizens of the Nightside. Taylor's irrepressible sarcasm and a twisted take on quotidian scenes compensate for a few too many solemn, self-important pronouncements and evil-overlord lectures. (Jan.)