4 Volume hardback set of British Birds by Archibald Thorburn
Published 1926 New Edition by Longmans Green & Co Ltd

Please see photos as a guide to the condition. 
The bindings are in good order without loose pages. There are some marks and light wear and sun discolouration to spines. I suspect these have spent most of their lives on a shelf. 
The pages have some brown spots and foxing. I have taken photos from all books to show examples of this, but these photos do not show all instances and it is of a similar nature throughout all books. 

Please ask for any further information and I will try to answer your questions