Wild foods are really important, I think. I like the idea of guerrilla planting too.
What I've harvested this year? Blackberries, plums, apples. Beech leaves are edible, but it took me too long to identify them this year, you're advised just to eat young ones, so I missed the season. The berries on hawthorn bushes can be made into a jam/chutney. Loads of oak trees round here, but I'm not going to bother with acorns, all that faffing about rinsing them, no way. I'd love to know a use for horse chestnuts, tho - I know you can make a medicinal tincture, but I'm wondering if there's any way to prepare them so that they're more like sweet chestnuts. And I'd love to find a sweet chestnut tree locally. Or a walnut tree. Mahonia berries are edible (and are found in many parks) could make a jam ... there's a wild cobnut tree locally, which I'll have a go at and let you know.
There's a parade of sloe trees local to me - I made chutney last year, but I had to put so much sugar in, I might as well have made jam. I'm going to try make just a fruit syrup this year, see if I can use it as a seasoning.
As far as a book is concerned, you couldn't do better than Alys Fowler, the redhead off Gardener's World, the title is The Thrifty Forager
The big gap in my education is the greens - miner's lettuce, good king henry, all that - between the cats and dogs weeing on them, and the pesticides that the landscaping people put down, I don't fancy them, not right now.