The Witch's Brew by Sigfried Trent.
The "Advanced Feats" series by Kobold Quarterly is just the stuff you're looking for, especially if you recently picked up a copy of Pathfinder's Advanced Player's Guide. Each of these dozen(ish) page articles are chock-full of a mix of the sort of "duh" feats that you can't believe haven't been published before, coupled with the "neato" feats that get you to re-examine class features in a new light. Sigfried Trent is the chief editor of The Netbook of Feats, & has a decade of mechanics balancing under his belt, & it shows; these aren't over-powered, but they definitely rank in the "useful" category-- no "Dodge" here. Trent attaches a commentary to eac feat, explaining why it works, how it might be useful, or why it isn't broken-- a level of transparency that any DM can appreciate. Each article ends with a few "Builds," a list of feats for your character to take at each applicable level in order to get the most bang for your buck. Personally, I like my characters to acquire feats organically, as they progress, but seeing a progression-- a chain reaction, so to speak-- is useful.
Secrets of the Alchemist starts off right out of the gate giving me ideas-- the examination of the class just struck a cord, making me visualize an alchemist who drops a poison gas bomb at his own feet to take advantage of his own immunity, & then proceeds to lobs bombs out of that, daring anyone to come try to engage him in melee. Hey, that guy is great, & the "Mighty Mutant" build for maximizing on the alchemist's Mutagens is a great Mister Hyde feat tree. Many of the feats for the alchemist made me think of my former player Gerd, who constantly bemoaned how badly off item creators had it; well, "Craft Anywhere" & its ilk are meant for people like him. I was sort of offended to see "Craftsman"-- just a boring old +2 to a skill feat? I thought things like "Accurate Assessment" & "Lighten Weapon" were weird & could be great character hooks. Something like Suicide Bomber is a cute ability but broken-- as a Dungeon Master it occurs to me only as fodder for "you see a goblin running at you. He seemed to be covered in Yule ornaments..." but I see too much potential for PC abuse, let alone making it a feat. Rules to cover it, yes-- because as the commentary says, your PCs will try it-- but a feat isn't the right spot for those rules.
I think that Secrets of the Alchemist makes me want to play the Alchemist class, but Witch's Brew seems the most useful to the class-- & to any class that resembles it. I don't like classes that are limited by spell list-- in fact, I hate spell lists entirely. They are a bottleneck, & limit fun rather than enable it. That being said, I'm not sure Hexes arn't entirely better than spells-- the downside seems to be that you can't apply metamagic, but who uses metamagic? Apart from a special case Quickened spell, that is. It just reminds me of how superior "Sudden Metamagic" feats were-- much preferred, thank you! Putting that aside, I think the feat tree of most interest are the ones pertaining to familiars & to countermagic-- both of which add a unique feel to the class. Familiars have always been under-exploited & the feats provided go a long way towards making them useful-- both from a mechanical standpoint & from a story perspective. While Secrets of the Alchemist has a great "jerk" power-- "Sundering Bomb"-- Witch's Brew has "Familiar Reincarnation" which is basically "save versus mean DM." The counterspell list goes a long way towards making it a viable character option, rather than what the author calls a "once in a character's lifetime event." I think my favorite feat in the list is "Signature Focus," which provides what is essential "Eschew Materials" for the more expensive spell components-- but does it in a way that is both balanced & flavorful.
Far & away, the best part of these supplements is how useful they might be to a range of characters. Sure, some of the feats are really only applicable to the specific classes-- some modify class features, for instance-- but others are just ripe to be picked up by any class. Want a slew of feats to buff your familiar, Wizards? Check out Witch's Brew. Want to juggle your inventory, item-heavy Rogue? Ah, then Secrets of the Alchemist is right up your alley. They are a tight grouping of neat powers which buff specialty classes, provide options to a wide range of PCs & have a few "snake hands" where unexpected rules blossom-- tattoo magic, damage reduction, using a healer's kit to actually heal damage, making ranged opportunity attack, that kind of thing. Above all it is an interesting series-- maybe you are playing an Inquisitor or a Summoner or some other class...in which case I'd be rubbing my hands together with glee, anticipating one of these bundles.