From Stornoway to Stonehaven, its prime aim is to "bring books to life" and it is "particularly committed to helping children and adults who are socially, economically and geographically disadvantaged".
"I don't know if there is such a thing as being 'geographically disadvantaged'," says Book Bus manager and driver Colin McEwan, "but remote areas which are not necessarily well-serviced with bookshops for children are a prime target for us."
I caught the bus at Kirkinner Primary School near Wigtown, in time to catch the eager faces pressed against the classroom windows. "In the islands and in small villages like this it really feels like we are the circus coming to town," says McEwan.
The "we" includes writer Janet Paisley who downs a quick coffee in the staffroom before heading into the hall to entertain some 40 P4 to P7 pupils who cheer her performance poetry and are soon up there themselves.
This is an interactive hour, cleverly structured (Paisley is a former teacher) but obviously good fun. Afterwards the children are brimming with ideas for poems.
"They just want to carry on from the ideas they got," says Jean Best, the headteacher. "It was really interesting that the boys came up with so many ideas during the session. They are less into books than the girls, and yet the majority who stood up with ideas for Janet were boys."
Class teacher Daine Peden, also stresses the "fun" element, adding "the children obviously loved it and it was in spoken Scots. That makes it more socially linked to the pupils' background."
Meanwhile the bus itself is swarming with kids and McEwan is clearly in his element. Something of a showman ("zany", says Paisley approvingly), he soon has the pupils at their ease. One minute he is giving an animated reading from a book, the next dealing with a request for "something on frogs".
Caught mid-performance by the three o'clock bell, he asks "Do you want to go?" The chorus resounds "No!" Between 300 and 400 children clamber aboard the bus in any week that it's on the road. "Sometimes we sell 100 books in a day, sometimes just in a week. A lot depends on how motivated the parents are, " says McEwan, recalling an incident on Orkney where a parent's car chased the bus to get more books as it was about to board the ferry.
"We encourage children to have fun with books, to instil the idea that books are fun," he adds. Janet Paisley says: "Poetry should walk and talk to be released from the page. That's why I love to do the verbal stuff with them and then they can go back to the classrooms to write. To me, that's bringing books alive."
Sponsored this year by Scottish Friendly Assurance, the Readiscovery Book Bus, which has been on the road for four years, always travels with a writer or illustrator and seems to meet a positive response wherever it goes.
Surely though, I ask, there must be occasions when it's not all beaming smiles?
"Sure," says McEwan. "I once got a bunch of boys who said they hated books. I did my routine anyway. But they told me to shut up - so that they could look at the books!" The Readiscovery Book Bus is a project of the Scottish Book Trust. Contact Janet Smyth 0131 221 1995.