I am a sucker for comic novels, and I don't mean "graphic novels/comic books", rather writing with a penchant for humor. My favorite series used to be the LotR but somehow the movies, which I initially loved and greatly anticipated, have neutered this series a little for me. Characters have been not so much ruined, but degraded, cheapened, watered down from where they were in my mind before Peter Jackson's little foray.
Now my favorite series is the scifi/humor classic Hitchhiker's Guide series, and Adams' followup Dirk Gently books. This too had its poor adaptations, but the 2005 movie was so, SO bad that it somehow has managed not to corrupt the books for me. I turned it off halfway through, it was so bloody awful. The BBC TV series was bad, but in a chintzy BBC special effects sort of way...somehow Big-Budget Awful is worse than low budget, we're-trying-but-we-just-don't-have-the-funds sort of awful.
Another great line of books that is not so much a series as a tangled web of interwoven novels and short stories are the two major canons of PG Wodehouse, the Jeeves books and the Blandings Castle books. Having exhausted the former I am enjoying of late some of the more prominent examples of the latter. Just finished Summer Lightning and Heavy Weather, both very good, and Leave it to Psmith being the first of the Blandings Castle series that I read, it remains a favorite.