Of all the many, many isekai series out there today, Overlord still continues to be a beloved addition to anime rather than some kind of reduction. While it is fast catching up to a near-ending light novel, if you finished up the Overlord anime, then you still have a few more light novels to explore if you cannot get enough.
After Overlord Season 4, you can pick up the light novels at Novel 15. Though, while the anime covers Novel 14, you may wish to go back and start with that one since it always kind of does a slap-dash job at adapting the light novels near the end of a season.
That said, Overlrod is confirmed to be ending with the 18th volume of the light novel, so not a ton left to read.
Overlord also has a manga, but it is well behind the anime.
*Updated for Season 4 end.
