Rory Shiner, One Forever: The transforming power of being in Christ, Matthias Media, 2013.
Fantastic little book. The most accessible, clear, helpful stuff I’ve read on union with Christ. Practical, punchy, brilliant illustrations. Can’t recommend it highly enough. If you can get your hands on a copy get one. Or get twenty to give away. And if you can’t get hold of it then you can read the first chapter [pdf] and listen to the original talks on which the book is based: