STRANGER... is... Strange.
Definitely part of the fuzzy fringe.
A lot of its success came from its shock value for the times.
I'm not sure what its reception might've been if it had surfaced now as an unpublished manuscript. (Probably a long debate right there.)
STRANGER is one Heinlein book I've never had any impulse to re-read, even though it was the *third* Heinlein book I ever read. (BEYOND THIS HORIZON and STARSHIP TROOPERS were first.)
Classifying it has always been problematic because the narrative is purposefully ambiguous. And because Heinlein was getting experimental and provocative (not always successfully) in his old age. (GLORY ROAD, FARNHAM'S FREEHOLD, NUMBER OF THE BEAST).