Congrats on the neat gift! Only thing I saw on the site was "fits books up to 15" tall". I'd assume the sides are around 1/2" thick and the over hang of the roof is around 1.5" - 2" on each side. The bottom is probably 1/2" thick also so subtract that from the outside dimensions given on the website and you should have a rough idea of what the inside is unless they have some additional support inside that we can't see from the pics on the website.
I have been wanting to put one up for quite a long time but live in an area where I'm afraid it would become a drug stash or an invitation for trouble. I still might do it someday because I think it could help someone and that would outweigh the downsides. I hope yours gets a lot of use!