The M is optional, he seemed to switch between using it and not doing so
Yeah, I know. He uses the M. for his sci-fi works and drops it for his literary works (although a couple of his literary works are borderline sci-fi). And yeah, The Wasp Factory is good. I've only read 3 Iain Banks books, and The Wasp Factory is the only one that I would whole-heartedly recommend.
As for his sci-fi:
Culture Novels
Consider Phlebas -- Fine, the major conflict in this gets namechecked a lot, but not essential reading
*The Player of Games -- Fun, but not essential
The State of the Art -- Short story collection and completely skippable
*Use of Weapons -- Probably his best sci-fi novel
Excession -- Slightly below The Player of Games
*Look to Windwards -- Probably 1 notch below Use of Weapons
Matter -- Has interesting ideas that are better examined in one of his other sci-fi books
Surface Detail -- Fine, but not that great
The Hydrogen Sonata -- Probably my least favorite book of his
Other sci-fi
*Inversions -- Quasi-Culture novel. About a planet that is at ~19th century technology and 2 people from a post-scarcity civilization's interactions with it. Better than I'm making it sound. 3rd favorite Iain M. Banks book.
*Against a Dark Background -- Fun but slight
*The Algebraist -- Fun, mostly slight, but my second favorite book by him.
*Transition -- An interesting look at a trans-dimensional cabal.
I recommend anything with a star.