The Book and The Spring is a mid level module for 1st Edition, though it seemingly borrows a bit from Adventures Dark and Deep, the idea of different monsters having different types of hit dice (d4 for small ones, up d12 for large ones). It's from Christopher Letzelter and his Anachronistes Press and is a bit unusual for two reasons. Firstly, it's mostly inspired by Sumerian culture, and secondly, it's split into two separate products — the module itself, and the map/supplement book.
I suppose it's also a bit unusual in the premise, which seems like the second half of the adventure (or campaign, even). The PCs are tasked with destroying an evil artifact, a book, by submersing it in a holy spring. However, the spring has dried up thanks to a magical curse by an evil king, now undead. But they can't kill him until they open his tomb, which requires finding keys.
Thankfully that can easily be ignored, since essentially you have a small sandbox setting with several sites for the PCs to explore, though three in particular are the largest and most detailed: A ruined palace, a tomb, and the ruins of a hanging gardens style structure. Each room is given a physical description, sometimes quite lengthy and often includes dimensions (which some people like but others seem to hate).
