The Deities and Demigods manual was indeed one of the favorite AD&D rule books for collectors. It was one of those books that you didn't need and would never use, but was simply too cool not to have. Plus back in the old days we bought everything TSR would put out that had a hardcover, even that Oriental Adventures and Wilderness Survival Guide nonsense.
The books really are not all that useful for the game, as most Dungeon Masters make up their own gods anyway. Those that don't tend to use the gods that come with one of the pregenerated campaign worlds (such as Dragonlance and Forgotten Realms). And in any case you aren't going to need the statistics of the gods.
These books are fairly useful in the fact that they are an overview of most dead pantheonic religions. They hit pretty much all the major real world ones, and some versions also feature some fictional ones as well.
The version featuring the Cthulhu and Melnibonean mythos is fairly rare, and commands a pretty decent price on Ebay. But the regular version is dirt cheap now and can be had for a few bucks. These are usually in pretty good shape, as they really didn't get used all that often. Now if you need an excuse to buy this book then just tell yourself that it is also a good general reference book on mythology in general.
Many people wonder why this book was ever produced in the first place. Well it seems that TSR had released an earlier book entitled Gods, Demi-Gods & Heroes. That book was the fourth supplement for the original "White Box" D&D rules. So they naturally updated it for Advanced Dungeons & Dragons. The original book was actually published as a last ditch effort to try to stop all the Monty Haul campaigns that were the norm almost everywhere in the mid 70s. By establishing that Thor himself was only level 25, it would make that fourtieth level warrior seem completely insane. They were successful in some ways, ultra high level play became much less popular, although low level groups carrying twice their weight in magic wands are still all too common.
So different versions of the god books were made. Eventually they decided to rename their series of "God Books" to Legends and Lore, which was probably an attempt to minimize any bad press. But Wizards of the Coast decided to go back to the original name for the 3rd Edition, which so many gamers seemed to prefer anyway.
Quick Reference to the "God Books"