This is a good addition to any 1st Edition AD&D library, particularly any one that is going to be set on a fantasy Earth. The book presents a number of well-known real mythoi (even if they are somewhat changed from their historical underpinnings for ease of gameplay -- but Schick and Kuntz do provide an extensive bibliography for the source material) including lesser-known pantheons such as the Native American and Finnish gods. There is even a section for King Arthur and his Knights of the Round Table (following Mallory's Morte d'Arthur, and not the post-Roman noble he probably was, but that's just a nitpick). Monsters and other heroes conected with the deities are also included.
But the most legendary point of the book is, of course, official AD&D stats for Cthulhu and others in the Lovecraft mythos -- and they are bruisers.
Don't let the deities' stat blocks fool you into thinking the gods are there like other monsters -- for the purpose of "kill it and take its stuff." These gods are GODS -- they have stats only to be used "if absolutely necessary." Any of these deities could reasonably be expected to kill a PC of almost any level within a round or two (and some, like Cthulhu, could even make short work of some of the toughest critters from the Monster Manual such as Demogorgon or Orcus). Use them sparingly as direct combat opponents (although sending Cthulhu [400 HP, 30 attacks (you read that right) of a damage level I don't recall, but obscenely high] against a "full of itself, 'let's rip the moustache off the king and raid his treasury'" party might just knock some sense into them <evil laugh>), instead trying to use them as either patrons or prime movers -- The PCs probably would never face Loki or Izumo, but might be fighting fire giants or Ogre Magi/Oni that the gods are directing.
One thing -- make sure to look carefully at the product description. If you want just any copy, any edition of this book (or the identical-but-for-a-different-cover-and-title Legends and Lore) are extremely serviciable. But if you're a collector, or are looking to use the 1st Edition stats for the Cthulhu or Melinebone (sp?) mythoi, be sure the product description states that it is the first edition -- after that (and perhaps a second printing), TSR decided to take the stats out rather than have to give a "thank you" credit to Chaosium. However, some later printings have the "thank you" note without the stats (they hadn't gotten around to changing the plates) -- therefore get the first printing and you're assured of Lovecraftian goodness.