Execute lsof -i:35729 to view the process occupying port 35729, and then find a process similar to the following:
node 93675 **** 23u IPv6 0x4ada9494f1377225 0t0 TCP *: 35729 (LISTEN)
Where 93675 is the ID of the process,
Then execute kill -9 93675 to kill the process, and then try gitbook serve