Do I need to read Dark Sun to understand the next books?
No, Dark Sun is a completely seperate story that fits between Mad Dogs and The Sleepwalker.

What is Dark Sun?
Dark Sun was originally published in the UK in 2008 as a 108 page CHERUB mini story.

After being out of print for almost five years, Dark Sun is now available again in a new edition, which includes the original story and some of Robert Muchamore's other CHERUB bonus stories.