Something new I discovered digging through the code for this venue, self employed gardeners get a 25% discount from the produce stand. There's some more information in there, but I've yet to decipher it all.
Edit: If you have the haggler LTR and join the gardener career, the price of the books goes down significantly. They are already very cheap compared to store prices. Given that there are 3 or 4 different books, and 10 of each, there's a hefty profit to be made. My 5 star sim, who also happened to get some celebrity discounts made about 75K just by buying all the books from the stands. It's an infinite money mine!