Books
Here you can find books that I highly recommend any software developer to read.
Clean Code
Clean code by Robert Martin is probably the single most influential book for me as a software engineer. I read it while in college, and it changed the way I write code tremendously. It especially opened my eyes on how I wrote comments.
You can find it on Amazon or on Bol.com
Working Effectively with Legacy Code
Working Effectively with Legacy Code by Michael Feathers is a very well written book on, well you guessed it, working with legacy code. It comes with a lot of practical tips on how to deal with your legacy, and turning it into something maintainable. This is a definite recommendation for anyone who works with legacy code.
