If you want to go more pricey, there are some group histories by Schiffer publishing as well as others that fill out the 38 story well too.
Possum, Clover & Hades-The 475th Fighter Group in World War II" by John Stanaway
Attack & Conquer-The 8th Fighter Group in WW2-by John Stanaway and Lawrence Hickey
An Escort of P38s-The 1st Fighter Group in WW2-by John Mullins
Adorimini-A History of the 82nd Fighter Group in World War II-By Steve Blake with John Stanaway
If you detect a trend, it's John Stanaway being the 38 guru he is.
The Osprey Aces Series are all by Stanaway
He's got a really wonderful book on the photo recce P38s of the 8th Photo Recon Squadron in WW2 as well with lots of photos of those F5s.
Bodie's book mentioned by Widewing is also a must have for the 38 fanatics.
Dan/Slack