This first photograph is from 1961 as you can tell from what the library assistant (or librarian) is wearing. She is so formal and professional - you couldn't possibly go anywhere without wearing a hat. The children are wearing the same sort of thing that I used to wear as a child of about their age in the 1960s. The mobileness of the vehicle appears to be that it contains a shelf of books to pull out and browse through. I do love the poster on the shelves announcing "Playground service" and the children are so serious.
I think that this photo taken five years later is iconic - it could be an artwork. Presumably it is the same vehicle, in its garage, waiting to spread books, happiness and literacy to the children of Providence. To me, who has never been to the USA and take my knowledge from films, it looks like it could easily turn into the Ghost Buster's vehicle.
As we can see here, however, it is a beautifuly illustrated bookmobile. It did turn into something...
It appears to have become a mobile art gallery, a good idea in itself, but I do feel a bit sad that in 2004 it is has become a glaring red, like a fire engine.
The album that is home to these pictures is at: https://www.flickr.com/photos/ppl_ri_images/albums/72157622565124845