A common problem, but one with a simple if not easy cure: don't go in to bookshops. It is the only way. You may go into shops that sell new books as long as you are not carrying any money, but not into second-hand ones (because the new books will still be there Next Time, but the second-hand ones won’t, so you risk running back home/to the office to collect your purse before That Book leaves with someone else). Don’t go into bookshops. Buy books for other people via Amazon (or from new bookshops with £7.99 exactly in your purse). Only look at abebooks.co.uk for specific things. Only go into bookshops having accepted that you will buy something once in there. Understand that this is how things must be for the rest of your life.

Alternatively, there is method (2). For every new book you buy, you must read two from the piles on the floor. It won’t resolve the problem completely, but it should help a bit.