Single gitbook from multiple repositories

Question

I want to use gibook to publish some documentation. My repo is something on the following lines

docs/
libs/
    sub-module-1
    sub-module-2

sub-module-1 looks a little like this

src/
docs/
    install.md
    blah.md

I am not sure if GitBook supports the ability to create a single gitbook where i can combine markdowns from both main repo and submodule repos.

I am also ofcourse also head scratching on how sync would work as main could be linking to a different commit hashes of submodules and not exactly clear if i were to modify sub-module-1/docs/install.md how would this get updated.

Nuclear option being , i move the docs out from submodules to the higher level main repo , but would like to keep them as close as possible. So exploring if there is a better way to organise this.