"His mother loves reading English books the most."
你没错！最 Is a superlative （-est) marker and preceeds the adj/verb to show its the -est.. 最大 biggest/最小 smallest.
Following your train of thought, 最喜欢 would be favourite, or "most liked"..
我最喜欢吃臭豆腐 I most like to eat stinky tofu/Stinky tofu is my favourite (thing to eat).
Follows much the same pattern with many other words or turns of phrase.
最好(的什么） The best (most good) something 最大(的国家) The biggest (most big) country 最高(的山) The tallest (most tall) mountain 最新(的书) The newest (most new) book 最近 Recently (most near/recent)
Check out the Chinese Grammar Wiki page for a way better attempt at explanation than mine! https://resources.allsetlearning.com/chinese/grammar/Superlative_%22zui%22