The best boiler maintenance operation book in China is difficult to answer. Only what suits you, where to find the best in the world, the level and requirements are different.

I think many practical problems can be solved with the 503-page Question and Answer of Industrial Boiler Maintenance and Reconstruction.

What you need to learn more in theory is: Practical Boiler Manual (Second Edition).

Edited by Lin Zonghu and Xu 1307

There is an electronic version. I can give it to you if you want.