Typically in Campaign Settings one expects an overview of the local gods so that a DM knows the abilities of their clergy in terms of game mechanics and background. In addition, a player may want to play a local priest where a campaign is set in that area.
Apparently, to understand gods of Ancient Osirion in terms of game play and background you need to buy Pathfinder Adventure Path #80: Empty Graves. This reference is made in the Bestiary Section of the Campaign Setting.
First, this is really a short change for a Campaign Setting. For example, the Dragon Empires Campaign Setting provides for a host of local deities as well as a discussion on how more well-known deities are viewed in the locality. Legacy of the Pharaohs has none of that. The purpose of a Campaign Setting is to give a DM the tools to run a campaign. Legacy of the Pharaohs is missing an important aspect for a campaign that many expect to feature ancient and esoteric mystery religions.
Second, there is also a practical issue here. Assuming one doesn't mind paying extra money on Adventure Path #80: Empty Graves, a la Wizards of the Coast, that Adventure doesn't come out for a few months. The consequence of this is that I'm probably not moving forward with this campaign (so no Mummy's Adventure Path) and move forward with another campaign idea without spending more money.