[Annual]

I vaguely remember The Gods Themselves by Asimov being a strong contender here, but it's been decades since I read it.

Spider Robinson's "Callahan's Crosstime Saloon" series had a story towards the end that blew my tiny little teenaged mind back in the 90's.

Octavia Butler, of course. Xenogenesis.

[stevenwoo]

The middle third of The Gods Themselves is so weird and different I thought it was a book printing error at first and I had to re-read some of it about six times to make it stick.