This is a page with some of my most helpful resources to date. I'll keep updating it and feel free to add your own in the comments page.
Complete Coach has a piece about the thinking self vs the observing self. Explains some key terms.
ACT (acceptance and commitment therapy) is explained really nicely in the first two chapters of Act Mindfully by Russ Harris.Worth a read as it explains some of the background in really easy-to-get terms.
The Happiness Trap is the book that started me off on this path of self improvement! Really worth a read through the website. Has a link to the first two chapters as well.
Dr Cliff (a personal blog) has a piece on the thinking self/observing self. Goes into some good detail.
Living Mindfully has a huge report on the topic. Gives evidence for mindfulness across a range of mental health problems and is a well referenced ol' chunk of reading.
Mindfulness and Sex is a book written by a Melbourne-based author and PhD candidate Owen Spear. He goes into some interesting detail about a topic we sometimes seek to avoid!
Act Mindfully is a good site with some links to helpful stuff. Have a look around and check it out.