I would say that is hard to have problems with the flavour of Paizo books and this one is very pleasant to read and useful in its informations about the game world.
I will follow Purplefixer example and do a section by section comment.
1: Magic of the Inner Sea: background material. A descriptive essay on some of the different styles of magic use in Golarion. Good section. 5/5
2. Variant magic:
False divine magic - Awesome. Great RP wise and for NPC, interesting for PC.
Fleshwarping - Good. Not my kind of stuff, but still good for NPC and PC alike.
Primal magic - Good. Not something that you should encounter every other day, but great to flesh out some adventure and character.
Riffle scroll - Nice. Underpowered for most PC, but very colourful. And a way to apply the silent metamagic feat to scrolls.
Shadowcasting - not the set of feats for you if you taking a feat that isn't the strongest in a book pain you, but more than decent for game use and very flavourful.
Tatto magic: Inscribe magical tatto - a feat that give you 11 item slots ..... Sure the exemplified tattos aren't the end of the world but it is a good item construction feat. And one I would allow people to use with the Master Artisan feat.
Thassilonian magic: A "second level" of specialization were you renounce to all uses of two specific schools of magic in exchange to the possibility to memorize twice the same spell in your specialization spell slot. More a NPC thing than a PC thing (especially as you need to make the choice at level 1, so you can't "learn" Thassilonian magic with an existing character and rarely a new character would have the chance to know it), but interesting nonetheless.
3: Magic schools: I have some doubt on one of the Guild benefits (probably a piece of text that should have been cut away slipped in) but nice. Usable factions rules and a good way to tie in characters with the game world. 4/5
4: Spellcasters of the Inner Sea: I disagree with Purplefixer. He examine the archetypes and prestige classes simply as power options for PC and so he dismiss them if they aren't more powerful than other options. I am more interested in the RP aspect of the classes and how they will interact with the world of Golarion.
- 2 new Oracle mysteries. Mostly geared for NPC, very flavourful.
- 19 archetypes. Most of them interesting and useful.
I will examine in particular the Tattoed Sorcerer archetype: a) it is a sorcerer archetype. You must compare what it do with other sorcerer options. b) familiar tatto: you get a familiar with a small added bonus as your first level ability. Slightly better than taking the arcane bloodline familiar. Varisian tatto. It substitute your eschew material feat with the capacity to cast spells in 1 school of magic at +1 CL. Neat. Bloodline tattoo. No cost for this ability and your bloodline spells are cast a +1 CL. Neat again. Create spell tatto. The level 7 bloodline feat is replaced with the ability to create 1 tatto that work like a automatically silent scroll at no cost. He can't have more than one "scroll" in existence at the same time. YMMV but it is power is on par with several feats. Enhanced Varisian tatto. One of the spells that is enhanced by your varisian tatto become usable once/day as a spell like ability. that replace your level 9 bloodline power. At a later level you can change the selected spell. For me having one specific spell of up to level 9 as a spell like ability hardly seem a weak ability.
Cypermage: essentially a "master of scrolls" prestige class. I see plenty of way to use his powers.
Divine scion: my less preferred option in this section of the book. probably powerful but not my stuff.
5: Spells: 39 spells. Some of them are earth shattering but none seem grossly unbalanced and generally they do very well their role of being setting specific spells. 5/5
General art and layout: excellent. 5/5
General vote 4.5/5, as there isn't the option for 4.5 stars, 5 stars.