Probably the best ones for a contemporary point of view would be the "Jane's" series, and the extra added bonus is that many of the individual annuals are reprinted quite regularly either on their own or as a single compendium.
Try typing in "Jane's" or something similar in something like Amazon or E-bay (Another good source of aviation miscellania), but be prepared to wade through a lot of information as you'll probably get 100's of matches that are not relevant.
A lot depends on what you're really looking for, but for hard facts Jane's is not brilliant since so little was known at the time they were originally printed about many Axis aircraft, and the author was severely limited as to what he could print about Allied types.
The good thing about Jane's books is that they are generally completely faithful re-prints with nothing altered or ammended, but because they're re-prints, they don't tend to cost the earth.
Other suggestions would be to check out a few Charity shops or second-hand bookshops next time you're down the high street, you'd be surprised at what sometimes turns up there, and it's usually quite cheap.
- The Great Planes : World War Two Warbirds
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