Yes, these are the Unseen Library editions - the first 18 books in the series published by BCA as Tony & poohcarrot have mentioned.
BCA has since become Books Direct
Until recently the last 5 or 6 books of the 18 were still available for around £5-£6 each, but I can't find any there now so I assume they are all gone.
They were published in sets of 3, the first set (COM, Light Fantastic & Equal Rights) in 2001 up untill Maskerade in 2007.
If you want to include the miss spelled Maskarade/Maskerade copy in your collection, there would be 19 to find.