Players Guide to Glorantha is pretty good.
There's a bit of guff about how to play characters, choosing sides and that kind of thing, so skip over that to the good bits.
There are new backgrounds, professions, spells, Legendary Abilities and Magic Items for The Godlearners, EWF, Orlanthi, Other Malkioni, Pelorians, Praxian and Pentian Nomads, Kralori, Teshnans, Vithelans, Pamaltelans, Aldryami, Uz, Mostali, Timinits, Ducks, Centaurs, Minotaurs, Hsunchen and Waertagi.
The backgrounds and professions look reasonable and are in addition to the standard ones from RQ or RQ Companion. The Character Generation gives stats, typical backgrounds/professions and a little bit of background and are good enough, although lacking in any great detail.
The Legendary Abilities are varied and interesting. The spells are generally good, although I am on the fence as to whether they should be part of cults or available as "Common" Magic for a culture. The magic items are useful and range from minor to major items.
Folk Magic is interesting but sketchy. There are sample spells and rules for generating spells and spell properties. They could be useful in games as they are not tied to a rune, automatically work and can be cast alongside other actions. There are a few restrictions, they are limited to 2 Magnitude and you can't learn a spell to increase a skill you don't have, both conditions I would quite happily ignore, and only work for one action/blow. But, if you want an easy +10% to a skill, then Folk Magic is the way to do it.
As to the quality of the book itself, I bought mine at Tentacles and it is good enough. It isn't falling apart, the bindings are quite strong, although two of the blank pages at the front and rear are glued together quite a way in, making them difficult to open, but they are blank so that isn't a problem. As to why there are effectively 5 blank pages at the front and 5 blank pages at the rear of the book, that's a different question. Perhaps they ran out of material to include, or they thoughtfully provided those pages for notes.
So, overall it's pretty good. It could have done with more detail, but it covers twenty cultures across the whole of Glorantha. Everything in it is immediately useful and allows people to play the PC species described.
If you want to play Minotaurs or Centaurs in any setting, not just Glorantha, this is worth buying just for that. If you want to use PCs/NPCs from any of the cultures described, this is worth buying. If you want cool spells, Legendary Abilities and magic Items for a non-Gloranthan game then this is worth buying. Otherwise, have a think about it and if you have some spare change then you should probably buy it.