I can’t find the original issue for this, but somewhere around 0.4.1 Gitbook was made not to unlink the target directory. This behavior remains in 0.4.2, but is broken again in 0.4.3 and above.

Here’s my file structure: https://github.com/macroid/macroid.github.io
So I build with gitbook build _src. Version 0.4.3 produces the following error:

c:\projects\macroid\macroid.github.io>gitbook build _src Starting build ... EBUSY, unlink 'c:\projects\macroid\macroid.github.io'