Levine's Guide is still the gold standard as far as general knife reading. 4th is best. 3 and 2 are worth it for the custom knife pictures, and info, if you collect old handmades.
"The Working Folding Knife" by Steven Dick was a very nice bit of knife writing.
The M H Cole military knives books are still the big books on military knives. Knife World's "Military Knives" book is a nice fun read too. Silvey's books are nice too. You can also read through the archives at
www.usmilitaryknives.com ( scroll down to the index link at the bottom ) , and get a good education on military stuff. Frank Trzaska is one of the best knife writers working today.
Knife World ( now Knife Magazine ) is a great source for very good knife books, they tend to still stock a lot of out of print titles -
https://www.knifemagazine.com/ .
The "Guns and Ammo Guidebook to Knives" (blue cover) is a great look at knife culture up until the 70's.
Ken Warner's Practical Knife Guide is a great read on knives up till the 70's too. Ken Warner was a good writer / editor on the subject. The Knives annuals were very good under his wing.
I note that Blade Magazine is offering CD-ROMs and PDF collections of their magazines now. I wish more magazines would do that. Love them or hate them, magazines were a big part of knife culture, and they covered a lot of stuff the books miss.
Norm Flayderman's The Bowie Knife book is a great book on the subject.
Ron Flook's writing is great on older British and European military knives.
D E Henry's Collins Bowies and Machetes is a book on the subject and machetes in general.
Those are some of the more general books I reach for when looking up knives.