As someone who struggles to maintain focus, I found Cal Newport's “Deep Work” to be instructive and useful.
The book was part theory, part practical guide to keeping distractions and unnecessary work at bay. Much of it was self-evident: don't tweet so much, but it was good hearing ideas surrounding why. The book is well researched, but much of the evidence backing Newport's ideas are collections of anecdotes. I love anecdotes, but collecting anecdotes is not the same as science.
I spent the past work week applying some of what I learned, and the early results are promising. Well-worth the read.