Don Unamuno by J Marias
Although written in the 1960s, this is the best general book written about Unamuno and his works. Nosick's book published by Twayne is also quite good, however, Marias' is undoubtedly the better of the two. Miguel de Unamuno is one of the great authors of the "Generation of '98" and his themes of the crises of self perception and human existence are universal and timeless.
T Frederick