There are very few books available discussing general aspects of ensemble methods. One of them is Ensemble methods from Seni, Elder and Grossmann. It provides a high level overview of ensemble learning. However, the book contains a lot of equations which make it hard to read from beginning until the end. You will rather pick a few sections and read them independently.
On one side, the book seems rather light for an academic audience (it only surfaces each topic). On the other side, it is too academic for industry practitionners. So it’s not fully clear who the target audience is.
To be noted issues regarding missing axis label on some pictures. Also the quality of certain pictures is really low. In conclusion, I would recommend it only if you need an overview of techniques in the field and are not scared of reading equations instead of plain English.