After I created my book in Leanpub, is it possible to switch from Leanpub editor to GitHub or Dropbox seamlessly?

Yes, if you have signed up for one of our paid monthly plans, you should be able to switch back and forth seamlessly (if you're on our free plan, you only have access to the In-Browser Editor).

You can change your plan here:

https://leanpub.com/author_dashboard/subscriptions