Novelists often have more than one publisher. It's not unknown for a book to be published by one company in the UK, another one in the USA and a third for what are known as airside editions - books for sale in airports. Then there will be contracts with large print and audio publishers too.
I find this site -
http://www.fantasticfiction.co.uk/g/sue-gee/ - a very good source of information (there's Nielsen Bookfind, but you have to pay for that) and usually up to date.
Having consulted NB, I see that Sue's most recent book was 'Reading in Bed' and was last published in 2008.