If you develop WordPress plugins, you need to read Professional WordPress Plugin Development by Brad Williams, Ozh Richard, and Justin Tadlock.
I first read this book around the beginning of April. I learned a lot of new things. And then promptly forgot it all. So yesterday and today I went through the book again and took notes. This book is around 500 pages and covers pretty much any topic you could wish for. I made sure to pay special attention to the security section since that is probably the most important feature of plugin development.
I found things that I want to do to improve my To-Do List plugin (and wrote them down this time) and things that I will keep in mind for my next plugin.
You need to know PHP to understand the code in this book (you can’t develop a plugin without knowing PHP anyway). I’m already familiar with a lot of WordPress’s hooks and functions and how WP works. I felt that this book was right at my skill level.
I recommend this book 100%.