The Irrisen book is not necessary. However, I'd suggest the Advanced Player's Handbook. There is also stuff from the Ultimate Combat book (Gunslinger) but that information you can get from the PRDs available on the Paizo website (scroll down on the left under Rules Archive).
So, I'm probably going to start this AP pretty soon. I'm an experienced DM but haven't run Pathfinder before, so I haven't bothered to acquire any of the books; the Android app and website have been perfectly sufficient for me as a player. I'm thinking this is the right time to start, though.
I'm picking up the core rulebook from Amazon within the next couple of days, but I wasn't sure what else I should get. Those of you who've run this particular AP, which books have you found most useful?
It looks like Bestiary 3 comes up a lot, with the first one close behind. How necessary would you say the Irrisen book is? Are there books that were useful later in the AP but wouldn't be necessary to have at the beginning?
(I already have the adventure path itself, obviously, and have just downloaded the free player's guide.)
I ran it starting with just the AP and the core book plus using the PRD and d20pfsrd.com.
I eventually got Irrisen, the map pack, Cities of Golarion, and Distant Worlds. Those provided additional flavor information but were not needed.
Irrisen is nice for the end of book 1 and book 2 but not necessary. Cities of Golarion is nice for book 2 with the Whitethrone information but not necessary. Distant Worlds is presumably similarly nice for Book 4 but not necessary.
They use a bunch of monsters from Bestiary 1, 2, 3, but you can get their stats online in the PRD or d20pfsrd. There are a few monsters from Tome of Horrors Complete, but their stats are done out here. There is a monster from Land of the Linnorm Kings, but its stats are spelled out in the module.
The APG and Ultimate magic are nice for witches but the rules info on them is all in the PRD.
There is a little bit on white witches (an archetype and prestige class) that is repeated in the Reign of Winter Player's Guide, these originally were printed in Inner Sea Magic and Paths of Prestige.
The guide also suggests some stuff from People of the North.
I'm finding the reign of winter pawns useful, as well as the bestiary and bestiary 2 pawns. (I'd like b3 pawns as well, but can't afford them.) The boxes include bases but the AP pawn sets don't.
The Reign of Winter Player's Guide contains both the Winter Witch archetype and the Winter Witch PrC.
While I recommend Inner Sea Magic anyways because it's neat, the materials out of it and Paths of Prestige that are used in Reign of Winter were reprinted in the Player's Guide.
I'm finding the reign of winter pawns useful, as well as the bestiary and bestiary 2 pawns. (I'd like b3 pawns as well, but can't afford them.) The boxes include bases but the AP pawn sets don't.
I'm sure that works out fantastically but its quite an investment.
I've been making my own counters for a little while now using google image searches and copying them down to battle mat size on word documents for printing out a night's worth of bad guys at a time. It works pretty well when I've gone off script on bad guy selection or description and have the time. Like when I described one of the Dawn Pipers as Heat Miser with a banjo, overalls, and a straw hat in a rocking chair on his hill of mud.
Yes, I make my own, too.
However, I do use the little box with NPC cards. I have noticed too many NPCs are confusing to players after a while, and the cards work great. I also make my own cards for NPCs who are not in the box.