What would be the best ways to keep records of secound-hand novels in a small bookshop?
It's pretty hard to argue with the efficiency of computerizing a book record system in an operation such as yours. Even though you are a small business, computers are now quite inexpensive, and inventory software can also be quite reasonable, given that you're really only looking for something to keep track (if I assume correctly here) of novels coming in as sales or trades, their condition, number, and resale price. I also assume you want that database to be easily updated and searchable.
You could also do a barcode system where you assign a specific barcode sticker to each incoming book, scan it once as it comes in, type in the title, condition and price, then when it scans out you automatically have that information and when you make the sale it will take it out of your inventory records for you.
Call any business that deals with small business technology needs and tell them your budget and needs. They are usually experts and finding what you need, in your price range, and the training you in how to use it. Good luck.