If you haven’t yet read Adaptation, you probably shouldn’t read Natural Selection until you do. But you can certainly read it between Adaptation and Inheritance, which is why it’s out now! (Inheritance will be released in the UK/Australia/New Zealand in September.)
It’s available as an ebook only (no print edition) for £1.99. I don’t have links to all online retailers, but here are a couple:
Hope you like it!