Hi, it is far from Hong Kong to Xiamen, the best way is by air. You can also go to Guangzhou or Shenzhen to take train to Xiamen. I just know Huatong Bus Company providing direct bus from HK to Xiamen. You'd better cantact with the bus company to ask about this information. You have no need to book it online. You can buy the ticket on the bus.

Hi Frank, thank you very much