Brett's book is great. Englishman Ron Flook has some good ones, The "London Knife Book" and "British and Commonwealth Military Knives" being the standouts. MH Cole's books still have something to offer, as do the Mike Silvey books. Bill Wright's Theater Made Knives book is interesting, as is DE Henry's Collins Bowies and Machetes. Also Gilkerson's "Boarder's Away I" dealing with pre-1850's Navel edged weapons.
Bowie Knives - Flayderman's "Bowie Knife" book, The Antique Bowie Knife book, and Zalesky's "A Sure Defense" are all solid.
General knowledge - Levine's Guide is still king probably the best general primer. Ken Warner's "Practical Book of Knives" series, and Steven Dick's "The Working Folding Knife" are also excellent.
Frank Trzaska's an excellent researcher and writer, but he mainly does it for the magazines (Knife World, and later Knife mainly). The best collection of some of these is "Knife World Presents Military Knives".
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My list meandered a bit, and some of those are a tiny bit special interest, and a few have gotten expensive price wise, but they're the ones I'd hand to some looking for a broad general and military knife knowledge.