Tree Crops: A Permanent Agriculture
First Published 1929, available free to download as it is out of copyright.
Tree Crops download page - various formats
First of all, I'm writing this without having read the complete text yet, I intend to add more info later.
The book is divided as follows:
Part One: The Philosophy
Part Two: Some facts about some crop trees
Part Three: Economics, Farm Applications and National Implications
The author, writing as he was early last century, was not a permaculturist as we would recognise today, but was a pragmatic observer of agricultural lands who had recognised the degradation to soils caused by the plow, especially on sloping lands. His thesis is that we could stabilise soils and improve yields by identifying, cultivating and exploiting some of the most prolifically cropping trees. Early on he discusses the demand for tree crops and acknowledges that human consumption is always going to be limited - with the best will in the world planting 1000 acres of pecans will totally saturate demand.
Instead he is advocating tree crops primarily as fodder for livestock to supplement grazing. This would probably be recognised today as silvopasture, but with an emphasis on fruit and nut yields either to be gathered and stored as fee or for livestock to collect themselves. Tree crops have many advantages over annuals, especially when livestock harvest for themselves.
Greater drought resistance
reliable yields year to year
season extenders - cropping at times when grasses or other forage are in decline
nitrogen fixing (eg honeylocust)
No-till farming so soil is preserved and perhaps enhanced
Improves yield from steep, marginal or rocky land.
At the end of Part One he is essentially trying to raise the profile of his cause and push for funding to found long term research institutes around the USA to develop and commercialize tree crops as has been done for annuals.