the entrance to the cave containing the book is relatively close to the bunker in the Fjords. there's a small puzzle with pressure plate to get past the barrier inside.
another good way to make money is, after upgrading all your runners, send out for ginseng/blackrose, use him to make your own health potions for free, thus saving money.
use the other 2 to collect mallechite and diamond, they sell for the highest at vendors.
keep in mind vendors eventually run out of money, but there are severeal npcs you can sell stuff to. The guy in the necromancy ring, sassan, there's a vendor in a small town on the cliffs full of dragon fanatics who has about 29k, there's a smith in the port where the zeppelin is docked. i'm not sure if there is a vendor at high hall or not. when you get to alleroth there are 2 vendors who have a lot of money, about 40k or so, i dont recall. in short. you should be able to collect enough gold to fully upgrade you runners and get a whole lot of skills trained at your trainer, as well as buying any items/potions from vendors for quite some time before you run out of cash.