Programs as Diagrams: From Categorical Computability to Computable Categories
amazon.comThe above post is a link to a recently released book on Amazon.
The book introduces a diagrammatic approach to the concept of a computer program and in so doing provides a concrete, pictorial view towards of a type of computational category. This, I feel, is useful to readers of HN because it provides an accessible path to category theory by its application to computation, something programmers implicitly understand.
From a cursory glance at the sample, the book seems very accessible for those with an undergraduate computing background. I would go as far as to say that if you are a bright, determined high-schooler with an interest in programming, you should be able to make some headway into the book.
The author's website has more details [1] and includes a link to a talk of his on related matters at The New York City Category Theory Seminar done earlier this year.