If you are currently using WordPress and you want to understand more about it, I highly recommend The WordPress Anthology.
It was written for developers. However, I am not a developer and I completely understood this book. The WordPress Anthology goes beyond the basics to give you a thorough overview everything WordPress. It shows you how to install and customize WordPress. It shows you how to dive into the inner mechanics of WordPress and make the code work the way you want. You can also explore the world of plugins, themes and APIs to add extra functionality. I even learned about something I had no idea you could do: Multsite! There is even a chapter on how to launch your application on a global scale with localization techniques and marketing tips.
As a WordPress user myself, I thought this book was great. I highly recommend it to other WordPress users!