I'd say the best book devoted specifically to the P-38 is Warren Bodie's "Lockheed P-38 Lightning". Bodie was a Lockheed engineer for a long time, and had access to both the Lockheed archives and the USAF (USAAC in WW II) archives. This book is no longer in print in hardcover, but is available used from Amazon. The soft cover has recently been reprinted.
Dean's "America's Hundred Thousand" is also excellent.
You will however note that in many books, you find that the later model P-38's were either not tested at War Emergency Power, or the data is not given.
If you get a copy of a book called "Allied Aircraft Piston Engines of World War II", you'll find there was a rather large horsepower increase under War Emergency Power in the later model P-38's. Another good book that covers the P-38 engines as well is "Vee's for Victory". That book is an indepth work on the Allison V1710 V-12 engine. You'll find that according to both of those books, the later engines in the P-38 should have pushed it to well over 440 MPH.
There are several other books available. Note that Martin Caiden's "The Forked Tail Devil' is not really well thought of, despite having plenty of anecdotal information and having legendary P-38 pilot Art Heiden as a contributor.
By the way, when you buy books, especially from Amazon, buy them through websites like
http://www.danosacm.com or
http://home.att.net/~C.C.Jordan/ , both of those sites are run and maintained by AH fliers. It does not cost you an extra cent, and it helps them keep their sites going.
