One of the greatest development tools you have is free, easy, and you already have all you need.
The tool is Reflection.
All the great managers I know take time each day to reflect on what they did and how they did it. What did they do well and how can they do better next time. Some of them will journal, which I highly recommend. But it can be time to just sit and think, think about what happened today, what could happen tomorrow and how do you feel about things.
Tom Peters, the great business author, talks about how the great leaders read. He also recommends that they have unscheduled time in your day to just think.
Bill Gates famously takes a week every year where he goes off in solitude and just reads - books, proposals, etc.