Hitchiker's Guide to the Galaxy... I read the first 4 of them at least 3 times each. Maybe 4. They were fun as a f*ck. The 5th book I only read twice. Not as fun. (But mostly harmless.)

Ringworld: twice

Evolution by Stephen Baxter: twice

Some star wars & star trek novels likely got read twice too.

Guess I'm officially a Sci-fi nerd.