And,
Aero Detail 10 (a combined book on the Me163 and the He162) is worth having, too.
(Try the usual spots eg Amazon and AbeBooks, although the available titles in the Aero Detail range are also on the Hobbylink Japan site.)
For single titles only, my vote also goes for both Valiant books - they really are good, regardless of whether you're being serious about details or just aiming to be better informed and nudge things a bit more than the kits might offer.
(I have both Aerodetail titles, too, and am glad I do - if you like photos of existing airframes (with useful captions), they're really handy and complement Valiant Wings.)
Cheers,
Matt