Probably the easiest way to find out what the books are worth is to find out what booksellers are charging for them. The easiest way to do that is to enter the details on the UK's largest secondhand books website:
http://www.abebooks.co.uk/
Some words of caution are appropriate:
1. Most old books are worth no more than about 10p to �1 each. There has to be something special about them before they become valuable.
2. Just because a book dealer quotes a high price for a book it doesn't mean that anyone is necessarily prepared to pay that price.
3. When a book does have a potentially high value (e.g. a first edition of an early work by a popular author), the condition is vitally important. Let's suppose that a particular first edition, in 'as new' condition is worth �100.
Just a small tear in the dust wrapper would probably take the price down to �80.
A tatty dust wrapper (but with the book in perfect condition) might reduce the price to �50.
If the book was perfect but the dust wrapper was missing, it might only be worth �25.
If the book (without a dust wrapper) had a few scuff marks to the cover, it might only be worth �15.
A little bit 'scruffier' and the price might be �8.
A 'reading copy' (i.e. complete but in generally poor condition) might be worth less than �5.
Same book, vastly different prices!
Chris