Caroline Lawrence's Roman Mysteries series is something I bang on about on here fairly frequently. They are children's adventure stories - mystery, crime solving etc., with a bit of history thrown in, and even a bit of Latin. IMO it's the modern alternative to the Famous Five. It's not sexist or racist, and the writing is much better than Enid Blyton. I would say most 6 yr-olds would enjoy it as a bedtime story, and fluent readers (say 8+) would manage it for themselves.
In addition Caroline Lawrence has a very childfriendly website \link{http://www.romanmysteries.com/indexflash.htm\here}. She responds directly to fanmail, and she also does a lot of touring/book signing etc., mostly in the South of England.
Anyway, the first book is \link{http://www.amazon.co.uk/exec/obidos/ASIN/1842550209/qid=1144868030/sr=1-1/ref=sr_1_2_1/026-6724133-9973231\The Thieves of Ostia} and Ottakars have got a special edition of it for just 99p atm 
Go and get yourself a bargain.