These days: Glen Cook's The Black Company series, which follows a mercenary army over the years on it's many campaigns, but is mostly about the things that happen during downtime, and how the company deals with political machinations and supernatural stuff.
I haven't seen anyone mention it, so I'll add Beowulf. It reads like an ancient version of Gaston's song from Beauty and the Beast - a sort of one-man-upmanship as he does more and more outrageous acts, ending with a one-man fight with a dragon.
My favorite "conversion" of this old book is perhaps The Thirteenth Warrior, which is like seeing the story behind how this tale came to be and "what really happened".
I'm glad others mentioned Black Company and Earthsea.
A third series I like a lot - that has some spectacular short stories that go with it - is also by Glen Cook. The Dread Empire. The writing at the beginning (the first book was in 1979, five years before Black Company) and ending (in 2012, the original manuscript was stolen and so he rewrote it and put in other stuff he was planning) catches me a bit at first, but then I get in the flow. I like how it presents an overarching world with a mix of sword and sorcery and something more like high fantasy, and lets you see a lot of characters. The scale of conflicts the characters deal with definitely keeps increasing. As far as influencing my D&D, there might be more in here that does than in the Black Company books.
I'm kind of curious how the ten short stories in An Empire Unacquainted with Defeat read to someone who hasn't read the series itself. The set of pirate stories don't particularly depend on knowing anything, and a few of the others seem like they have enough introduction with them. One of the stories collected in it was the first fiction he published under his own name back in 1971.
On a semi-unrelated note, I also love his sci-fi work set in the Star Fishers universe. One of the great short stories set there ("In the Wind" from 1975) gave us part of the Plain of Fear in the Black Company.
It's a great example of exciting and dramatic adventures by characters who are really just blindly stumbling forward, without any great quest that comes to a neatly wrapped up resolution.
And it's just full of a range of wonderful fantastical places where the many varied encounters happen.