I'm not going to knock your purchase as this is one great book, but the rarity of this item may actually be one of the biggest D&D myths out there. Upon reading this thread, I did a quick search on E-bay and found 8 copies available right now (and those are just the ones I could peg as being "first prints" without too much work).
When TSR wanted to do a "gods" book, they found that the Cthulhu mythos was in public domain at that time, and thought that gave them the right to publish it. What they weren't aware of was that all previously published material was still owned by book publisher Arkham House and that they had contracted Chaosium to make a role playing game based on the Cthulhu myth. Also around this time, TSR had contacted Michael Moorcock about putting the Melnibonean mythos in the book. He gave them permission, but what he failed to tell them was that he also had entered into a contract with Chaosium to make a role playing game based on his Elric books. Unaware of these two facts, TSR went ahead and made the first printing of Deities & Demigods. It was then that they were contacted by lawyers for Chaosium.
The two companies sat down and came to an agreement. Chaosium would allow TSR to continue printing the book as is (with the two mythos in question allowed) provided they acknowledged Chaosium's license. With the next print run (the second), TSR added the following line to the bottom of the preface on pg. 4, "Special thanks are also given to Chaosium, Inc. for permission to use the material found in the Cthulhu Mythos and the Melnibonean Mythos."
With the next printing (the third), TSR decided it didn't want to give free advertising to a competing gaming company and just removed the Cthulhu and Melnibonean mythos.
Like I said, I don't want to "beat down" on your find, it's a great book and you paid an appropriate price given the condition, so enjoy. I just want to dispel this myth because I paid considerably more for my copy and was very upset when I learned that these are no harder to find than the mono-colored modules.
peace,
Rich
PS - printing information on Dieties & Demigods was taken from
http://www.theacaeum.com