What distinguishes esoteric buddhism from other buddhist sects that were prominent in japan?

Esoteric Buddhism has an element of mystery not seen to the same degree in other Buddhist sects. The teachings are very philosophical and the bond between the practicing individual(believer) and Buddha himself is very special. Since this is a unique part of Buddhism the teachings are not available to the general public in the form of books and such that the other sects have available at their disposal.