Why 5 stars?
Well first off, this is free. Free. For free one might expect a cheap PDF with no art or background. This has both.
Also, the players guide does not give any of the story away, at all. Everything is left up to the GM and players to discover and hash out.
I thought it also gives detailed informative tips about developing each race and class so that they could be more successful.
Did I say it was free? This is one of those things that you can use if you want, or if you don't think it gives enough detail, as a GM, add more information

The art which opens this text isn’t very good, the intro which sets the scene is. This is why I run a game in and around Sargava, one of the most exciting regions in Golarion, too long neglected in Paizo’s early days. On page 6, claiming the fighter is suited to the dangers of Sargava is not actually true.

Wearing any armour lowers fort saves against heat and fatigue, and Sargava is a very hot climate. The typical fighter can be stuck on too many penalties if he adopts heavy armour and shield, and wear out swiftly without multiple great forts. Smartly it does suggest less armour and not to take great reductions to mobility. As a DM who has run games in Sargava for many months prior to this adventure path, a fighter without diplomacy, a common fate, can also get into more fights than they can handle. The players I ran an adventure game for were a little too combat focused with not enough skills or even moderate charisma to keep them socially afloat. Sargava and the Mwangi expanse are dangerous, but it is not pit fighting day-in, day-out, and a great fort and diplomacy can do wonders for survival. Oh and don’t forget to take survival and nature, that can’t be emphasised enough.

The campaign traits add great flair and a reason for a character to be in-bound for Sargava.

I give the small book a three, almost a four, for doing what it sets out to do well.

This Player's Guide does exactly what its supposed to: it not only helps my group structure their PCs, but it gets them revved up to play Serpent's Skull.

The download is free, so I didn't feel entitled to getting too much out of this file. If anything, it gives the DM the best gift of getting their player's excited for the game and for their PCs.

Mmmm, my love affair for Pathfinder is increasing...

The payer guide is 'only' 12 pages, contains information about races with some recommendations, the 'book' gives you also some info on the starting point,but does not show very much information. The good thing is that the starting point of this adventure opens a path for practical every race or class to start. so there is not much info needed. However, only 3 stars because i like to dream ahead and some pictures or drawings from some hidden temple or old buried tombe would have made it more 'book'