If you ask people for their recommended reading on agile, you'll probably get a list of books on agile process, blogs by agile experts, and social media recommendations to follow that will be full of agile coaches. And, I wouldn't discourage you from reading any of them. In fact, I should probably read a few more myself. But, agile is rooted in good leadership. So, for me, the starting point for any agile reading is understanding how to create better leaders.
The most influential book for me is Drive by Daniel Pink. Daniel lays out the three intrinsic motivators for all humans. They are shockingly simple and yet can be incredibly hard to achieve at the same time.
Autonomy - We all want to feel in control of our own destiny. We want to believe we can influence the things we do, that we can make a difference.
Mastery - We all need to feel that we are masters at our craft. That we are respected for our abilities.
Purpose - We want to do whatever it is for a higher meaning. It should be for something bigger than ourselves.
The first two concepts align incredibly well with the agile manifesto. Much of the manifesto is about taking control from the processes and giving it to the individual. Pair programming and code reviews are closely aligned with increasing mastery. The book covers this material and the supporting evidence in great detail. If you don't have time for the book, Dan does have a TED Talk. And if you can't even spare 18 minutes to hear the material, there is also a 9 minute RSA presentation on the material.
D. Michael Abrashoff was commanding officer of the USS Benfold. From this experience he wrote It's Your Ship which is fascinating because he demonstrates how he could create an agile organization within the strict command of the Navy. Think your company has strict process and policies? More strict than the military?
Abrashoff's story is remarkable in both the way he ran his ship and the results they achieved. He put the responsibility of the ship in the hands of his crew. One interesting parallel to agile was his training which involved a primary, secondary, and tertiary person for every single task. Training was done by pairing individuals until there were 3 people available to do any task on the ship. Imagine if every software component had 3 people that were confident in changing or fixing it.
While neither of these books are specifically about agile development, they are about creating an agile culture. In Switch by Chip and Dan Heath, you learn that change requires two things. Both the emotional being and the thinking being must be aligned. They use the analogy of the elephant and the rider. The rider knows where he wants to go but if the elephant isn't in agreement, there will be a wrestling match the rider will ultimately lose. This is equally true for organizations. To move to agile, you have to not only reach the intellect but the emotion. To do that, you have to appeal to the intrinsic motivators and Abrashoff shows you can do that within the structure of any organization.