Colin Ward, the visionary British anarchist who died on February 11 at the age of 85, was an inspiring advocate for mutual action and a staunch defender of squatters.
Most relevant to squatters was his recent work on what he called 'housing's hidden history,' Cotters and Squatters, published by Five Leaves.
Here's a review of that book, from The Independent
And here's Anarchism as a Theory of Organization, an essay from 1966.
You can read more about him on Wikipedia, in a profile that ran in The Guardian a few years back, and in this generous appreciation from Next Left.