1. create new book repo and push it to gitlab.
2. go to http://git.jkr3.com/<userid>/<your.book>/hooks.
3. copy and paste http://book.jkr3.com/webhook in URL.
4. make sure Trigger > Push events is checked
5. and then press Add Webhook button
6. Done.

you may need add deploy Key to your repo

so, each time your push new commite to gitlab, it will automatic rebuild it. you can press the Test Hook button to rebuilding your books immediately.