The cover to the left is from the 1980 edition of the Monster & Treasure Assortment, which is in fact a compilation of three earlier sets published by TSR between 1977 and 1978. Taken together, these sets provide 100 monsters and 100 treasures scaled for dungeon levels 1 through 9, the idea being that a referee could roll percentile dice in order to randomly stock his dungeon. While this might sound like an uninteresting product, it's actually a rather fascinating one, at least from a historical perspective.
One of the reasons I find it so fascinating is that the 1980 edition, which is the one I own, doesn't just compile the contents of the three sets from which it's made; it revises -- selectively. Consider that, when it was first released in 1977, there was OD&D, the AD&D Monster Manual, and the Holmes Basic Set. While there's a high degree of compatibility between these three variations on D&D, they're not identical to one another. By 1978, when the later installments of this series was released, the AD&D Players Handbook was available, adding yet another possible source of rules variations. The 1980 compilation predates the release of the Moldvay Basic Rules, but, looking through the monster and treasure listings, you'll see entries that seem to reflect the contents of Moldvay. In the end, it the Monster & Treasure Assortment seems to use a hodgepodge of rules sets rather than any single one.
This jumbled character is apparent too when you look at, for example, the format used in the monster entries. Each of the monsters has an abbreviation of "AL" followed by a number. According to the book's introduction, "AL" is attack level and it's THAC9, that is, the number needed by the monster in question to hit an unarmored opponent in OD&D and its descendant games (but not post-PHB AD&D). Saving throw entries are clearly Moldvay-derived, as there are references to racial classes. But then there are also references to monsters like Type I and Type III demons, multiclass halflings, and Will o' Wisps that once again make it clear that the Monster & Treasure Assortment was never fully updated in 1980 to a single, consistent rules set, instead being a mixture of elements from several different games.
Now, I don't think this hampers the utility of the product at all. I've gotten lots of use out of this book in my Dwimmermount campaign and I used it for years beforehand. Still, it makes one wonder what TSR was thinking when they released it. My suspicion is that the growing popularity of D&D demanded that the company release product -- any product -- at a prodigious rate in order to keep up with demand. This resulted in some earlier materials being re-released under new covers since there simply weren't enough new materials to sell at this stage. Three decades later, we sometimes forget how big D&D was and I have little doubt TSR was caught as flatfooted by this turn of events as anyone.
When I first bought this product, I honestly didn't notice that the book didn't line up consistently with any single version of D&D. I spoke many "dialects" of the game and could freely translate between them. Indeed, my own campaigns used a mixture of rules from Holmes, AD&D, and Moldvay. We (largely) didn't see them as hugely different from one another mechanically and freely used products intended for one with the others. Nowadays, I actually find the quirkiness of the Monster & Treasure Assortment charming. I also think it's a testament to just how similar all these descendants of the LBBs really were -- much like the clones of today.