I can only speak to North America but a lot of the science applies.
https://www.sibleyguides.com/about/the-sibley-guide-to-bird-life-behavior/
https://academy.allaboutbirds.org/textbook/
https://www.amazon.com/Birders-Handbook-Natural-History-American/dp/0671659898
And then
https://www.amazon.com/Complete-Birds-World-Species-Illustrated/dp/0691193924
https://www.amazon.com/Birders-Handbook-Natural-History-American/dp/0671659898
Is a good one for North America. Half bird book and half natural history and science.
The Birder's Handbook is a great ornithology primer.