Last year, I bought Paul M. Jones' book Modernizing Legacy Applications In PHP and used it really push my PHP and my websites into OO and much better design. I am still working through that book and pushing my code into better and better ways.
Paul M. Jones has published another book on LeanPub called Solving The N+1 Problem In PHP. The recommended price is $8, but you can pay more for it if you want. I bought it when it came out, and have started working through. I have some currently intractable problems with the blog software, and I am hoping to work a solution with this book.