This setting sort of takes a Ravenloft-esque world, then dumps a bucket of Lovecraftian mythos over it. This means a lot of changes to the typical Pathfinder game. For starters there are different core races, five of them. First is the Bhriota, actually a human culture rather than a separate per se. The Bhriota are savages with a religion based on worship of the Old Ones (Lovecraftian gods). The Cambion are next and are not what I was expecting. A cambion is usually a demon/human hybrid and so I was expecting a version of the tiefling. However, this is a Cthulhu mythos game and so they are part human and part Something Else. These are not badass looking tieflings but really hideous, warped Quasimodo types. In fact they are a lot like the character Quasimodo, even down to sometimes being taken in and raised by the church. Then there are the Dhampir, a human/vampire mix. In this setting the Dhampir are an echo of the time when vampire lords ruled, but later wiped out when the Spawn of the Old Ones returned. The romni are
Each race comes with racial traits, great setting-specific alternate alternate traits, and complete class-specific adventurer notes and favored class options. The authors are not shy about using the adventurer notes to make it clear that certain classes are extremely rare, limited by gender, etc. based on the setting. This doesn't prevent you from using them and GMs will find this useful to help build NPCs in tune with the setting.
The usual elf, dwarf, gnome, and halfling races are noted as having existed in great numbers but now reduced to scattered remnants after being almost totally exterminated by the return of the Spawn of the Old Ones and the Great Cleansing by the vindari. Although acknowledged, they are definitely not given the same in-depth treatment as the five major races for the setting. I suppose you if you play one you're almost doing a "Last of the Mohicans" type character. The book also notes that drow, fetchlings, ratfolk, changelings,nagaji, and strix would be appropriate for the setting, with just a short paragraph on each.
In addition to all the core and base classes for Pathfinder, Shadows Over Vathak also adds five new ones specifically designed for this setting: Apostle (a chosen messenger of the One True God), Blade Singer (fighter specializing in thrown bladed weapons), Eldritch Conjuror (summoning), Rifleer, and Sword Dancer (wandering specialized blade fighter using a dancing trance). The setting also adds archetypes for all the original Pathfinder classes and the five new ones.
The book then has a section of 59 feats, some of which are very setting specific, others more general. I was almost disappointed to see so many feats. One of the problems areas of D&D 3.5 and Pathfinder is rules bloat. And feats is one of the areas most prone to bloat up with just too much material. I would have preferred to see a tighter list with only flavorful setting-specific feats on it. It really has gotten to the point where I skip over any new feats listing and avoid buying any feat books.
Equipment gets its own section. It starts out with a lot of firearms, including muzzle-loading black powder guns and several which use metal cartridges. The bit on the revolver notes "these weapons are typically carried by vindari marshals". Marshals with six-guns, really? I found that a bit jarring. However, once I got to the firearms I understood better how the vindari invaders were able to sweep all before them in the Great Cleansing. There are also some cool armor types and equipment, plus romni "smoking weed" with varieties which produce various effects on the smoker.
Like most setting books, Shadows Over Vathak has a section on new spells. This is another area of the 3.5/Pathfinder rules with a strong tendency for bloat. A lot of these are quite good, with clear inspiration from the setting. But as with the feats I wished they'd kept new spells to an absolute minimum, skipping the non-specific ones.
Next, at last, we get to the gazetteer chapter. I love a good flavorful setting and after the mostly crunchy (and sometimes bloat-inducing) bits we finally get to the setting itself. First comes the calendar: names of days of the week and months and main festivals. I really liked that these were all in keeping with the pseudo-european flavor of the setting. For instance, October becomes Octombrie. They're easy to pick up but definitely remind you of the setting. Each of the major regions is treated in turn, with the towns and cities each given a stat block and enough text to characterize it while leaving room for DM creativity. I love a good fantasy campaign gazetteer and this entire section was great. Time after time I'd read about a location which just cried out to have an adventure set in it.
Religion is also covered. Unlike many fantasy settings this one is quite bipolar, with the Church of the One True God versus the Cults of the Old Ones. This is a bit stark, but well in keeping with the setting which has overtones of a vampire movie. The Church of the One True God (Lawful Good) is similar to the Catholic Church in Spain in the Renaissance, but with new sub-groups. The Cults of the Old Ones has a short entry on each of the Old Ones (all of which are chaotic, evil, or both), then longer treatments of the various cults which follow them. Again this section is solid setting design and well usable.
There is then a really good DM section. I was glad to see that Shadows over Vathak skipped the usual "What is a role-playing game" section which wastes page count in most books. This section really gets down into providing a lot of lists, especially ones you can roll on, of elements custom designed to not only help DMs gin up adventures but also convey the creepy elements of the setting. The information for generating villages was particularly good. This section also has a lot of delightfully gross diseases to toss into the game. A nice surprise was the weather section. I almost skipped over this until I spotted some of the interesting rules for fog and stopped for a better look. In a horror setting weather is very important for providing tone and atmosphere and Shadows Over Vathak takes the time to not only include it but make it relevant.
Next is a section on forbidden lore--almost a must with a Lovecraft mythos game. There are rules on insanity, various books and items and their effects on users. The book finishes up with monsters, mostly Lovecraftian, and monster templates.
So the bottom line is that if you're interested in mixing Call of Cthulhu with D&D with Ravenloft then this is the book for you. I think it's very usable (my Wednesday group is likely to experience it in the future) and definitely worth the money.