You can hear an interview with his biographer here in which it’s mentioned that he wanted to film a komodo dragon and an ape going at it, and how that led to King Kong.
In the book The Making of King Kong Goldner (who worked on the film) and Turner say it was based on a desire to film a gorilla fighting a Komodo Dragon, but no one ever mentions any particular expedition or book being at the root of that.
You can also find elements of the Kong story in Chang, one of Cooper and Schoedsack’s earlier films, in which there is a great deal of buildup about a long-unseen and mysterious animal.(It turns out to be a baby elephant)
In the book The Girl in the Hairy Paw, one of the essayists makes the case that one of H. Rider Haggard’s lesser-known stories resembles Kong in a lot of ways, right down to the sacrificed virgin (although the “monster” turns out to be a guy dressed up in outlandish robes).