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).
The tomb is pretty much what you expect from a tomb, partially filled with undead (plus a few gnolls) as well as tricks and traps. The ultimate villain here is a spellcasting mummy. The last, the ruins of a hanging garden, is where the Sumerian flavor really shows. It's the lair of a lamia and her followers.
There are essentially three factions at play; The cultists (including the gnolls), a group of descendants of the original city who moved elsewhere, and the lamia and her followers. There's not a huge amount of interaction between the three, but it could be developed.
While I would say the bulk of the monsters are standard ones, taken from the three AD&D monster books, there's also twelve new ones, detailed in the supplemental book. These include the sirrush, girtablili (scorpion men), and gallu, which you've all probably heard of. It also adds the ghastly Dahak, which is like a chimera with skeletal heads, a bat demon, a golem, and some more monsters, lizards and carnivorous cactii.
As mentioned, this is made up into two PDFs - one with the adventure keys and the other with supplemental information, like new monsters, illustrations for players, and of course the maps. This actually makes some sense, at least on PC, because otherwise I often have to use two different PDF readers to view a PDF so I can see the map and the encounters at the same time (since I could never figure out how to open two copies of the same PDF in the same PDF reader). They are nice looking, with largely original art (I think, at least I haven't seen it) and in color in most cases.
The maps are especially well done, looking like something that came out of a university textbook on archaeology.
All in all though, it's a must buy for 1st Edition fans and those looking for something a bit different.