For British literature...
If you enjoyed watching Harry Potter, you could read the His Dark Materials series:
http://en.wikipedia.org/wiki/His_Dark_Materials
These are fantasy children's books like Harry Potter, and were also best sellers, but are slightly more complicated in vocabulary.
The Edge Chronicles and The Chronicles of Narnia are also similar.
Terry Pratchett also wrote a huge number of good books that are easy to read, of many different genres.
If you want more adult or more varied fiction you could try novels by Ian McEwan, Iain Banks, Neil Gaiman, P.D. James, Agatha Christie, Roald Dahl or Douglas Adams.
All these authors use reasonably simple, modern language. Older books (19th century and early) generally improve your vocabulary more, but they can be challenging to read.