Perhaps you picked up the omnibus edition? They don't have those here across the pond (at least not in any of the bookstores I frequent) but I see this in Wikipedia - looks like 2 books per omnibus. The first book you got must be different than these.
Quote:
* Into the Nightside (Nightside Omnibus 1) (Solaris Books, 2008), ISBN 1-844-16642-2 :A collection of the first and second Nightside novels.
* Haunting the Nightside (Nightside Omnibus 2) (Solaris Books September 1, 2008), ISBN 1-844-16638-4 :A collection of the third and fourth Nightside novels.
* The Dark Heart of the Nightside (Solaris Books October 6, 2008), ISBN 1-844-16635-X :A collection of the fifth and sixth Nightside novels.
* Damned if you Do in the Nightside (Nightside Omnibus 4) (Solaris Books, 2009), :A collection of the seventh and eighth Nightside novels.
* Just Another Judgement Day in the Nightside (Nightside Omnibus 5) (Solaris Books, 2010), :A collection of the ninth and tenth Nightside novels.