So the other dm in our circle is planning a pirate-themed home brew game and is looking for a map of the inner sea that highlights major trade routes, along with some information about what kinds of goods are moved along these routes. If anyone knows offhand what books would offer the best information the help would be appreciated.
I doubt there's a map that has this, you'd probably have to read through a lot of books to create this kinda thing
They don't exist. Rather, smart kings hired (at great expense, though still cheaper than a large sailing ship) powerful Nexian mages to create permanent teleportation circles that connect major cities and serve as the replacement for weeks of overland travel.
It might be worth asking this question in the Pathfinder Campaign Setting forum too. There are some scarily fanatical people there who know far too much about Golarion trivia there.
Why don't you have a look at the Skull and Shackles Adventure Path.
It's address the question of trade routes specifically, but it is an AP all about piracy in the place most well known for pirates on Golarion.
I don't have all the Campaign Setting books, but I sure have a lot of them. As far as I know, there are no maps showing trade routes of the Inner Sea. For that matter, there's scant little about Golarion's economy on the scale of nations or international trade. Because that's not really the focus of the game.
You'll need to infer where the trade routes are by looking at the maps and figuring out what is produced where, and how they can get their products and resources to market. Since much of Golarion is an analogue of the real world, try looking at real maps of trade routes in the middle ages, Renaissance, and Age of Sail, and then try your best to apply those real-world routes to Golarion.
There are mentions of a few trade routes in Inner Sea World Guide.
Aside from that, individual Campaign Setting guides may mention imports or exports, but it is not information that I have seen mentioned consistently.
The Pathfinder Wiki notes the Imports and Exports where they have been mentioned (though I am not sure how up to date that information is).
Overland trade routes are better covered (though even that is not much), such as the Path of Agabhei (sp?) connecting Varisia to Tian-Xia over the Crown of the World (prominent in Jade Regent), and there is mention of a cross-Casmaron trade route terminating in Qadira, if I recall correctly.
Honestly, the details are probably left to the individual GM to define as needed. The very abstract nature of trade in the Caravan Rules from Jade Regent, and the equally abstract Kingdom-level Trade Routes in Ultimate Campaign, make the details unnecessary (mostly).
Varisia, Birthplace Of Legends has a map of caravan routes (that includes Riddleport) on the inside back cover.
Most of the major routes going to/through Absalom probably count as major trade routes, since it's basically in the middle of the Inner Sea Region and is the biggest city there.
[Trade routes] don't exist. Rather, smart kings hired (at great expense, though still cheaper than a large sailing ship) powerful Nexian mages to create permanent teleportation circles that connect major cities and serve as the replacement for weeks of overland travel.
Or at least they should have done that, but there's enough traffic in the Inner Sea to maintain piracy. Then again, are there any 17th+ level wizards around?