This is one of those questions that needs to be a lot more specific as to the collecting area if we are talking about one or two books.
German daggers, uniforms, helmets, etc, all have their own answers. As do Japanese, US, and other fields of militaria collecting.
For general info on German daggers, I would buy Tom Johnson's Volume 1 (in print) or John Angolia's "Daggers, Bayonets, and Fighting knives of Hitler's Germany" which is long out of print. Both are 35 years or old and have a few glitches, but they are the only reasonably accurate books that covert the whole range of daggers.
For specialized books on daggers, Tom Wittmann's Army, Navy, Luft, and SS books are valued in-depth references. For NPEA, Ron Weinand's great new book is the one to own.
If your interest is railroad eagles, John Meda - or "JR" on this forum wrote what I am told is a great reference.
Hope this helps,
Dave
