Notoriously, if briefly, jailed for "conspiracy to corrupt public morals" at the 1971 Oz magazine trial, he plans to spend his retirement on a "Napoleonic madness". Mr Dennis is to use his wealth to plant 30,000 acres of traditional broad-leaved trees in south Warwickshire.
Mr Dennis - who received a lesser sentence than his two co-defendants because the judge decided he was "very much less intelligent than them" - has spent the past three decades building a formidable publishing business.
He is probably best known for Maxim, the huge-selling men's lifestyle magazine. But he is also behind the monthly Computer Shopper, Europe's heaviest magazine, which runs, at times, to 900 pages.
Yet he describes trees as one of his lifelong loves, along with women and books. He says he came to them in a "Damascene conversion" in a Soho square a quarter of a century ago.
In a book he is to publish next week on British trees, Silva by Archie Miles (Felix Dennis/ Ebury Press, pounds 30), he tells how he was walking "aimless and miserable" when he came across four huge hornbeams in Golden Square.
"Enormous, brooding and magnificent in their snowladen outlines ... they seemed one of the most beautiful sights I had ever witnessed," he writes in the book designed by Oz's artistic genius, Jon Goodchild, who died this summer. "From that day on I became obsessed with trees ..."
It is, he admits, "an odd obsession" for a publisher, "an irony that is with me every day". But now worth "between pounds 100m and pounds 500m", he is planning to pay the trees back.
"I have no children and have already put out trust funds for the loved ones in my life. So, I have this mad idea this is the best thing I could do with it."
He has already made a start: 10 years ago he bought 700 acres in Warwickshire and has planted hedges and woods, and opened up footpaths for local people. "It's magic. The bloody animals, birds, beetles and everything keep flooding back."
He reckons his forest will cost pounds 1bn to pounds 1.2 bn, and plans to give at least pounds 200m of his own money and raise the rest. He says it will be ecologically varied and have free public access. "It will never be called the Forest of Dennis," he acknowledges. "But it's that in my mind."
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