Welcome to Skull Tower (The Arduin Grimoire Volume II), softback supplement for Original D&D, by Dave Hargrave
The second of the three books that came to be known as The Arduin Trilogy, from 1978. Sort of an RPG in its own right, but most commonly seen as a series of supplements for Original D&D – new rules, new classes, new monsters, new magic items, lots of new charts, just a lot of additional stuff. (A lot of it stuff that you didn’t know you needed at the time, like the female attributes chart…)
It’s a bit…odd. But that was how RPGs were played back in the 1970s. And the author, Dave Hargrave, was somewhat opinionated (something that comes through in his writing). Still, something of a collector’s item, and very rare outside of the US.