Great sharing, I share my bit as well.
It is possible you buy those books twice per level (at least on low levels <10)

Restocking
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As soon as you enter Cyseal, go straight to recruit Madora and Jahan. They will be at lv 3. By the time I did that when I was still level 2 and there was a XP gap between them and my hunters of around 3000 EXP. If you skip all the exp thing at the beginning (tutorial cave, exploration exp, Ishmashell) you should have an even larger gap, giving more room to manage your EXP gain and separate levelling. So after the Teller is unlocked, you can restock every time using Madora/Jahan first then your hunters, offering 4 books per level, provided you have the money.

Bartering
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As for the money matter, the high price is a big matter. Here below is the bartering mechanism so that you can calculate and manage how the money should be spent.

General mark-up of vendors = 150%.
This is the base price for vendors having 0 attitude and buyer having 0 bartering skill.

This means the original price of each book from the Teller = 5000 (worth) x (1 + 150%) = 12500

Each bartering level lowers mark-up by 20%, so at lv 5 bartering the total mark-up will be 50%; therefore price of each book = 5000 x (1 + 50%) = 7500

Each attitude point lowers mark-up by 0.5%, so at 100 attitude the remaining 50% mark-up will be cancelled out. At max attitude and lv 5 bartering you can buy and sell at the actual values of all items.

Levelling bartering is simple, I suggest getting 2 lv and let the gears/personality(egoistical) do the rest. 3 ability points is absolutely affordable and worth considering what you can get in return in the future.

As for raising attitude:
1 charisma = 1 reputation = 5 attitude
so be sure to equip whatever that can boost your charisma when bartering (eg. smelly panty)
I have noticed this seems to related to vendors' level.
Price of 1 attitude = 10 x vendor level.
At least I'm sure you'll need 200 gold to earn 1 attitude from the Teller.
I spent a one-time gift of 14000 coins to get her attitude to 100 from 30, saving me 1750 each, which will be paid off after buying 8 books.

For Cyseal vendors (in the market, Esmeralda, Conrad the merchant on the ship) I'm not sure, I think they do level up eventually until they reach lv 5.
You can earn 50 attitude from the fish vendor by stopping the fish-thief.

Some extra tips on selling and giving gifts.
1. Give gifts in gold coins as much as possible. Attitude gain is based on the selling prices of items. 1 gold coin always equal 1 gold coin.
2. Same as above, buy books using gold coins as much as possible unless you have reached/surpassed lv 5 bartering and 100 attitude.
3. Sell non-craftable items first (paintings, golden cup, golden plate, books, etc)
4. Some crafted items will level with the player, even battered cooking pot and branches, yielding higher worths and thus prices.
5. Save up knives (check if that's gold) and daggers, they can be recrafted on anvil level with the player.
6. Sword, axes and some other weapons can be melted into bars and be made into other weapons that will level with the player and probably better prices.
7. Remember to boost weapons/armor before selling.
8. Crafted food has higher value than ingredients (of course)