[This is part of a series on the tools I used to write my PhD. Check out the other parts here.]
During the period I was working most intensely on my PhD writeup, I read over 100 books. I put that number out there not as a confrontation, but as an illustration that reading is important to ensure you don't get lost in a small box of your own creation. Judging purely from my own experience and from sporadic conversations with a loose handful of fellow PhD candidates, this can be a real problem.
Reading widely and about issues and problems wholly unrelated to your field of study is, I believe, the hallmark of a curious mind. If I meet someone for the first time and I'm assessing their work, I'm far more likely to be interested in the last ten books they've read than many other data points. Even the fact that someone is taking time to read, and to read diversely, is an important indicator for me.
I think I can date my adoption of this books-and-ideas-for-cross-fertilisation to when I read Steven Johnson's book Where Good Ideas Come From. He makes a strong case for a more deliberate approach to how you develop and cultivate ideas in your thinking life. (The book is short and highly suggestive of specific approaches to work. I'd recommend it if this kind of thing interests you).
I've found that things that I don't track and monitor tend to fall beside the wayside. Hence Goodreads and Beeminder and a number of other tracking tools. Goodreads allows you to set how many books you want to read each year and then keeps a convenient little widget reminding your how far ahead or behind you are of your goal. If you want a bit more of a 'sting' for non-compliance, you can hook up Beeminder and you'll be kept honest that way.
Reading books on unrelated topics was something I would do in the afternoons or evenings after my Four Perfect Hours. The time would be mine and I could read without any sense of guilt or that I wasn't making progress on my PhD writeup. No, I'd done my work in the morning, so now I could read to my heart's content.
Encounters with books are encounters with other ideas, other minds. It refreshes your approach and your sense of perspective -- both so important for your PhD. Give it a try! See how you can add in some reading time to your daily routine. Even 30 minutes before bed each evening adds up in the end.