OK, it's not exactly a manual, actually a handbook, The KML Handbook by Josie Wernecke. Josie is the Google tech writer who wrote the KML 2.1 and 2.2 documentation, and also helped write the KML 2.2 OGC Specification. So she knows what she's talking about!
The KML Handbook is the most complete treatment of KML in print. It explains all the various elements and features of KML. It also examines both well known topics like Regionation, and lesser known topics like View Based Refresh. It is also the only book on KML officially endorsed by Google.
The book is available for pre-sale from Amazon and O'Reilly, and should be available for immediate purchase soon. So for all those would-be Santa Trackers, and anyone else in the geographic world, you now have a great holiday gift.
Link - from Google Geo Developers Blog