George Bradshaw produced his railway guide from 1839 onwards, it was the first ever national guide to a railway network and continued in print updated every year until 1961. The early guides are very collectable and as stated above have been used by the BBC as the basis of a series of programmes looking at the changes that have occured in Britain and across Europe in the last 100 years. In Victorian England it would be normal to have a copy of Bradshaw if you needed to get anywhere by train (or in later editions ferry) as it was the only place you could get the details of what train went where and when.
The Bradshaw in Raising Steam is a clear reference to George as she is doing exactly what he set out to do, that is explaining where to go, why to go there and most importantly how to get there.
My guess is that this will be the third of the Pearson books, tied into Discworld and with the Pratchett name on the cover but with very little actually produced by him inside.