I just started reading Coingecko's book on Decentralised Finance. How to Defi. I am currently about half way and I can recommend it to anyone who is interested in DeFi but doesn't yet now a lot about it.
In the book they also offer a lot of examples and guides to some of the more popular DeFi products and they link to a lot of interesting articles in the book. So for anyone who wants to dive deeper into a specific topic they can without having to go though a lot of trouble finding relevant articles.
The book itself isn't to long at 212 pages but if you calculate in all the articles they reference I would say you can about triple that figure.