Hi Georgia
By last novel, I assume you mean latest novel - as Patrick McGrath is to the best of my knowledge still alive and writing!
If it's not for sale on a huge internet bookseller such as Amazon then the chances are that it's not in print or has a different title. If you believe the book is out of print then try a used book database such as http://used.addall.com/
You should also check to see if it's published with a different name in English - check the description on the Italian version and then check the description on his books in English - http://www.fantasticfiction.co.uk/m/patrick-mcgrath/ is a useful place to start - and his latest book listed there is Trauma (2008) - so that might be a good place to start.
Good luck!
Davina Morgan-Witts