Can anyone suggest me good theory books? I'm really trying to understand music theory so that I can write better songs, and the internet is not really doing it for me so can anyone suggest me some good books from www.ebay.co.uk ??

Btw I'm heavy metal fan but I don't think that matters... Cuz theory's theory right?

Theory for the Contemporary Guitarist

by Guy Capuzzo.

Great information through a course-book type feel. It's got quiz style pages that allow you to apply what you've learned through reading and test how well you know it. I'd definitely recommend it as a starting point.
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The complete idiots guide to music theory is pretty good. It comes with an ear training cd as well.

Seeing as you're writing songs; read the idiots guide, then read Mark Levines - Jazz Theory, then read a classical melody writing book (they're simply called "melody writing" by X authod), then read a classical harmony book (I'd recommend Walter Pistons - Harmony) and then read a counterpoint book (I'd recommend Kent Kennans - Counterpoint).
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Hey what about the The Complete Idiot's Guide to Music Composition ?? is that good too?