The Sea of Fertility (豊饒の海, Hōjō no Umi) is Mishima's masterpiece, written at the end of his career—with the second volume, culminating with the seppuku of Isao—prefiguring his own ending. It is named after the lunar sea Mare Fecunditatis.
It consists of four novels in a series that stretches from 1912 to 1975, following life events of Honda, a lawyer who got to experience a mystical, deep connection with his friend Kiyoaki, who dies in the first novel (in the snow of spring):
In the four successive novels, Honda gets to experience weird, irrational bonds with various people whom he connects to Kioyaki, as if they were his reincarnation.
Spring Snow is noted for its beautiful recreation of Japan in the Taishō period. Runaway Horses is an ode to Mishima himself, prefiguring his own final act. I haven't read (yet) the two others.