As a general rule, if you don't hear back don't assume permission. While you did this for helpful reasons an author of an article may still prefer not to have things republished so they can control updates, error correction and future republishing of the work. Gitbook does look great though, thanks for pointing it out.
Glad you liked it! GitBook is a pretty cool tool I'd heard about before; and when I started reading Nick's posts, I knew it was the best time to try it haha.