Sources:

A) Book just on the Four Knughts

1) Four Knights: Move by Move by Cyrus Lakdawala. Everyman Chess, 2012. 352 p. . White's point of view.

2) The Four Knights Game by Andrey Obodchuk. New in Chess, 2011. 240 p. . The Spanish 4 Knights from White's point of view.

3) The Four Knights by Jan Pinski. Everyman Chess, 2009. 192 p. .

4) New Ideas in the Four Knights by John Nunn, 1993.

5) An Unbeatable White Repertoire after 1. e4 e5 2. Nf3 by Larry Evans. Chess Digest, 1988. 152p.

B) Books with one or more chapters on the Four Knights

1) Beating the Open Games Beating the Open Games by Mihail Marin. Quality 2007. 331p. Chapters on The Four Knights (22p.), The Glek (10p.) and The Belgrade Gambit (6p.)

2) Play the Open Games as Black by John Emms. Gambit, 2000. 224 p. Chapters on The Belgrade Gambit (6p.), The Scotch Four Knight (15p.) and The Spanish Four Knights (17p.)