The book was first printed in 1980 by TSR, inc. and is now printed by Wizards of the Coast in Seattle, WA. The printing history of this book is fascinating due to it's several transformations leading to the present 3rd edition which has greatly evolved from the original. The 3rd edition printing will have a cover that looks like the one below:
The CTHULHU MYTHOS was originally from H.P.Lovecraft and related fiction. MELNIBONIAN MYTHOS was from Micheal Moorcock's Elic novels. These mythos were not public domain so could not be used with out permission!
Due to the copyright problems TSR had two options, they could give credit the proper people and include a special "thank you" to Chaosium for both the CTHULHU and MELNIBONIAN information. This special thank you did appear in the second printing of the book.
When the time came for the third printing of the book TSR decided it was two much work to mention everybody involved so subsequently took out the CTHULHU and MELNIBONIAN information.
The way to determine that it is a first printing that both the CTHULHU and MELNIBONIAN MYTHOS will be included and has 144 pages. The later printings contain only 128 pages do to the deleted mythos.