New garden bed started

I'm putting a new garden bed in this year -- it's going to be 9' x 10', and I was initially going to make it 12" high (2 6" boards) but decided for three tiers of 6" boards instead, for a deeper bed that would be good for growing stuff like parsnips and carrots.

The second picture here shows the dirt I'm dealing with in my back yard. It's heavy clay (ignore the gravel for the moment, that was pea stone we added), with spud-sized rocks interspersed throughout. When it's hard you can barely get a pickaxe into it -- believe me, I know this from experience -- and when you do, you hit a rock, which jars your whole arm.

I've read that clay can actually be pretty rich in minerals, and if you put humus into it you can get great garden soil, but I really don't know where to start aside from just randomly adding peat moss, compost and sand. I happened to remember seeing a book among the ones we picked up from K's late dad's collection on the subject. Rummaged around a bit and found the book; it's called "The Gardener's Guide to Better Soil", and it's by one of my favorite garden writers, Gene Logsdon! I'm also reading "The Contrary Farmer's Invitation to Gardening" at the moment, which is pretty inspirational, even if I can't implement some of his ideas (like keeping chickens) in my little patch of suburbia. Hopefully I'll get a better grasp on what amendments the soil needs here, rather than just randomly throwing stuff into the dirt and hoping it works. It's clear that some of my veggie beds are doing better than others; going about this more systematically would be a good idea.