Note: I recently noticed that I missed this one back when I reviewed the adventures in Mummy's Mask. I started this review around the time that I had to disappear for a while due to overwork. Since the book only exists in PDF form and doesn't have a physical copy (unless you print it out yourself), it's never been on my “to review” shelf, and so I completely forgot that I had never finished and posted it. At any rate, I have now finished it and so here it is.
One thing that has often (perhaps surprisingly) varied in adventure path Player's Guides is how much direct advice they give on character creation. Some offer only brief descriptions of how each character class might fit in. Others go into more detail, offering suggestions on specific feats, spells, skills, and other abilities that would work well for their particular adventure paths. The Mummy's Mask Player's Guide is one of these more detailed ones, spending several pages on suggested options for characters.
I like the layout chosen for this as well. Instead of going through classes in alphabetical order, it looks at categories, such as suggested archetypes, animal companions, favoured enemies, and so on. It also covers the languages that will be useful, common religions, and even suggestions for skills, feats, and traits beyond the campaign traits that are introduced later in the book. Another very useful inclusion is some suggested gear to purchase at character creation. Since much of the first adventure will involve searching through ancient tombs and buildings, this includes the types of things that are useful in dungeon crawling, such as candles, chalk, magnifying glasses, and rope. Players would be wise to take note of what's on the list and try to acquire as much of it as they can.
Where the guide is less detailed is with the background and origin of the characters, but this is not all that surprising, as characters in Mummy's Mask can come from pretty much anywhere and have just about any background a player can dream up. It's also left to players to decide how their characters come together and form a group as the adventure path doesn't have a moment when they meet. They are assumed to already be a group when the first adventure begins. One bit of character background the book does cover, though, is party names, since this is important to the first adventure. There is a sidebar with some options. Groups can choose one of these or use them as a guide to creating their own name.
The Player's Guide continues with an overview of Wati, the city where the first adventure takes place. Although the PCs don't necessarily come from this city, they will be spending a lot of time here, and it's useful for players to have an overview of information that their characters can pick up fairly easily. This is doubly useful if their characters actually come from Wati. There is also a map of Wati later on the final page of the book.
After this come the campaign traits for the adventure path. There are some rather interesting ones here, including Mummy Cursed (which is does not actually give your character a curse, but rather determines that one of your ancestors was cursed and your character is now more resistant to such curses) and Resurrected (which establishes that your character has already died at some point in the past and been brought back to life). There's a lot of flavour to the traits and they'll help characters fit well into the adventure path. However, they are also not absolutely necessary. Characters won't be at any disadvantage if they don't take one of these traits.
The book concludes with an overview of some of the environmental rules from the Core Rulebook. Since Osirion (where the entirety of the adventure path is set) is mostly desert, it's important that players (and gamemasters) be familiar with the rules for heat dangers, thirst and starvation, sandstorms, and so on.
The second-last page of the book is a sheet of hex paper that will be useful for exploring the PCs will conduct later in the adventure path. It's good to see the Player's Guide preparing players for later segments of the adventure path, not just the opening. With anything like this, there's a fine line between necessary information and spoiling too much. Here, it doesn't give too much away. There's just a mention that there will be some exploring later, and players might find the hex paper useful.
Overall, I would consider the Mummy's Mask Player's Guide to be one of the best adventure path Player's Guides so far. It gets straight to the point and provides players with the information that they need to create characters that will fit into the Mummy's Mask Adventure Path.