I studied for an Advanced Diploma in music, which gave me a great understanding of chord construction/harmony/keys etc. It also helped me to read music a little better. However, I'd been playing for more than 25 years before I took the plunge and put in some serious study. I've got half a dozen songwriting books but don't use them much. I'm curious, just in case there's a secret inside one of them. You never know, they might impart a little gem of knowledge that you can't do without. Mosly, I learned how to write by listening to The Beatles, Paul Simon, Don McLean, Neil Finn, Roddy Frame, Nick Heyward, Lloyd Cole and my favorite writer Paddy McAloon. I'm sure there are many others that have contributed to my understanding but these are the people that have influenced me more than any others!