Horse Books

Firstly not sure if this is in the right place so feel free to move
We've found a few horse books in the attic, was going to shove them in the carboot stuff at a £1 a book the realised they hadn't actually been read or in some cases even the pages havent been turned. (In fact some of them look like they might be the first editions from the 1990s).
Does anyone have an idea of where to find a realistic price I've looked on Amazon but there such a wide descripancy and I know zero about horses.
Thanks

Just thought I'd add the prices seem to go from a few pounds up t0 £70 for the same book. Want rid but dont want to let them go too cheaply.