Kent Beck 
Tidy First? [PDF ebook] 

Tidying up messy software is a must. And that means breaking up the code to make it more readable, and using guard clauses and helping functions to make it understandable. In this practical guide, author Kent Beck, creator of Extreme Programming and pioneer of software patterns, suggests when and where you might apply tidyings in your code.Instead of trying to master tidying all at once, this book lets you try out a few examples that make sense for your problem. If you have a big function containing many lines of code, you’ll learn how to logically divide it into smaller chunks. Along the way, you’ll learn the theory behind software design: coupling, cohesion, discounted cash flows, and optionality.