For Spanish speaking knife enthusiasts, I think all of Abel Domenech's books are a must.
If you're interested in self defense, Janich's "Knife Fighting", or any of his work for that matter, would be useful.
Bernard Levine's "Guide to knives and their values" (and his pocket knife book as well), Wayne Goddard's "$50 Knife shop", Bill Bagwell's "Bowies, big knives and the best battle blades", Loveless' & Barney's "How to make knives", Hrisoulas' "The pattern welded blade", Bergland's "Norwegian knife making", Fowler's "Knife talk: the art and science of knife making" (and "Knife talk II"), etc.
There are also several cool brand specific books about Spyderco, Victorinox, Case and others.