once I update my source file, the server stop with following error:

Stopping server
events.js:160
      throw er; // Unhandled 'error' event
      ^

Error: EPERM: operation not permitted, lstat 'd:\polymer-2.0\_book'
    at Error (native)

after that , the _book has been deleted.

So, what's wrong with gitbook? It's unbearable to restart the server by hand again and again

Some info maybe helpful:

OS: Windows 10 x64

node -v
v6.11.1

gitbook -V
CLI version: 2.3.1
GitBook version: 3.2.2