Ensemble learning is a kind of state-of-the-art machine learning method. It is well known that an ensemble is usually more accurate than a single learner, and ensemble methods have already achieved great success in many real-world tasks.
Here, I recommend a book of Ensemble Learning written by Professor Zhi-Hua Zhou (https://cs.nju.edu.cn/zhouzh/).
Here is the book on Amazon:
@book{zhou2012ensemble, title={Ensemble methods: foundations and algorithms}, author={Zhou, Zhi-Hua}, year={2012}, publisher={CRC press} }
I know it is very a very expensive book and it is illegal to share it openly. But, I guess nowadays, it is a common sense that every book has its pdf version online right? :P. That is the reason why we love the internet. But it is unfair for the authors to spend a lot of time and energy to write a good book, I suggest readers do the right things.
Gook luck with your study!
Here is a small tip for your convenience: Google the book.