"Il s'était assis, pour lire un livre."
Are you perhaps mixing the verbs "to seat" and "to sit"? If not, I guess you could use "he had seated himself" but that would mean, to me, something more formal, as if he was in a ceremony. E.g., "he had seated himself, disregarding the usher".
Anyway, the verb "s'asseoir" translates to "to sit".