Dry moist or wet is a condition of soil only after water has been added.

Thus I suspect your clay soils are the same as elsewhere. Wet to sloppy following a good rainfall, medium a week or so later, and dry if not watered for an extended time.

So, the plant encyclopedias and other books which list the moisture conditions under which an individual plant will thrive--dry, moist, wet -- are referring to how dry, moist or wet you should maintain the soil with a combo of rainfall and irrigation.

Soil moisture levels are the same whether you are in Maryland or Arizona. There are many things that affect your soil moisture levels, amount of rainfall, drainage, water table and while you can easily find swampy land in Maryland you are unlkely to find much in Arizona, although it is there.
An evenly moist soil determined by taking a handfull of soil and squeezing it tightly, will clump together but alos fall apart fairly easily, aka a soil in good tilth. A wet soil will do the same thing except it will not fall apart when poked and a dry soil will not hold together.

For Christmas of 2001, Santa gave me a $14 soil probe that was supposed to read pH and moisture - a special at the local garden shop, so several of my gardening neighbors had them as well. The summer of 2002 was the "exceptional drought" here, with trees at 5% moisture (kiln dried lumber is 10%)

Anyway, the soil was so dry that you couldn't actually insert the probe without bending it, and us gardeners used to stand around and joke about this.

The vast majority of plants will prefer well draining soils that are moisture retentive. This sounds like an oxymoron but is really a rather easy situation to achieve. Slopes, even those with primarily clay soils, tend to be well draining by virtue of their topography. But both moisture retention and improved drainage can be achieved by the incorporation of lots of organic matter before planting. Of course the actual moisture level will be determined by the amount of rainfall or irrigation provided.

FWIW, most of your state has a higher annual rainfall than many parts of Washington and Oregon. It's a common mistake many outsiders make thinking that we are under water much of the year :-)) We don't get all that much rain - it just comes in very small amounts over an extended period. And unlike many other areas of the country, we are the driest in summer when most folks are getting soaked on a regular basis (extended droughts excluded).

What you want to do, is plant things that are native to your area. They will be able to cope with the normal water content of your soil without your having to fuss over exactly what it is.

Clay soil holds water longer than other soils do. You probably get a fair amount of rain distributed throughout the year, so I don't see any reason to worry about the exact water content of your soil. The slope will help the clay from staying water logged too long.

And - plant encyclopedias tend to be written for your neck of the woods, not ours. Most American gardening books are pretty useless here on the Pacific Coast, they're written for the humid eastern half of the country, where you are, with a nod to the special conditions of the midwest and the southeast, but pretty clueless about the intermountain west and the pacific coast. I tell novice gardeners here to NOT buy any nationally-distributed garden books, but to stick with either British books (similar climate to the PNW) or books written specifically for the Pacific Coast or PNW. So when these encyclopedias you're looking at say dry moist wet whatever, they mean YOUR dry moist wet whatever. Not ours.

You may be making this project way more complicated than it really is. Look at your rainfall distribution over the course of the typical year. So long as you are getting a few inches of rain a month in the growing season, you aren't likely to have dry soil, especially with clay. Make sure what rain you get is allowed to infiltrate rather than run off the surface. Planting trees and shrubs will help the rain infiltrate rather than run off. Mulching with tree chips or leaf litter will help greatly too.

We get actually very dry soil here in the rainy PNW. We tend to have gravelly soils in many areas, and no rain in the middle of summer, so our soil can get bone dry for a couple of months. But in the winter I am grateful for that gravelly soil, otherwise I would be under water all winter long. Here in Olympia we get 55-65" a year, from September through June, nothing in July or August most years. We get nearly twice as much rain as Seattle does. It's just an unremitting drizzle for 9 months, rather than scattered downpours like the eastern half of the country. An inch a day is a lot for us. In the native forests, very little rain reaches the ground sometimes! the forest canopy can intercept most of a typical winter rain event of up to an inch in 24 hrs. After a few days of that though it does start to dribble through the canopy. Remove the trees, and it's a different story - lots of runoff and erosion. That's why I think you should plant native trees on your land.

People writing for print media like to recommend "rich, well drained soil". One day on a whim I Googled "rich, well drained soil" and got 39,000 ghits. Then I Googled "rich, poorly drained soil" and got three ghits. The point being that writers often develop linguistic styles. It is easy to write dry or moist or wet. While the "measurement of soil moisture", which produces 59,000 ghits is rather cumbersome, if you are a writer. A berkley.edu site says "The soil moisture content may be expressed by weight as the ratio of the mass of water present ..." you get the idea, people had rather write, and read dry, moist or wet.