There is currently not a third tier to be bought. Once your standing is high enough they become your vassal, adding a portion of their power value to yours. That is how all factions operate, although most cannot be made your vassal at the time. I'm sure that will be added in future updates. As for purchasing slaves from other factions, I don't believe there are any outside of Kymanto and the two slave markets.
If you're asking where the actual bookstore itself is, it's the location labeled "BOOKSTORE" in Whitehaven. The quest begins after you ask the apparently prudish owner of the bookstore why she stocks the terribly obscene Art of Paine II, and her shifty response makes you think something strange is going on. Progress it by assigning a valid girl to the bookstore through her Jobs & Titles menu, under Stores.
It's available right from day 1, so if you're desperate to see the complete introduction text, you can just create a throwaway character and visit the Bookstore.
You can assign guards and workers through the ACTIONS page of the NPC. Darkmere Mill/Farm produces wood, which you can use or sell. You can also assign guards from the Darkmere Mill page -- up at the top, hover over the little icons and you'll get their info popups.
EDIT: Oops, the little icons on the Mill page won't be there unless you also have Fort Sera. The icons send the guards from Fort Sera if you have guards there (or from the other properties if you own them).
You need Science over 100 and the laptop from the mastersphere, as well as the companion techie (make sure they are top-notch fighter also)-- at least, that's what it took for me. Sorry, I didn't write down the passcode. This is an example where Grimdark needs to make multiple routes to victory because this is one of the encounters that forces the player to develop in a specific way or lose the game.
