"Is this wood brittle?" No.

"Meaning will the screws shatter the wood?" No.

"Do I need a pilot hole? And if so, what size and how deep?"

I would. Just a little smaller than the screw and same depth.

Ditto Joseph, pilot holes are easy insurance against splitting wood.

No not brittle but its extremely dense/hard - its old growth pine, which is nothing like the farmed wood of today which is weak and splintery from having been fed growth accelerants. So IMHO thats 1 reason why you would need pilot holes, cos it could be ery difficult to drive a screw in otherwise. Pretty wood!

As a general rule, I never put a screw into any wood that I care about. Why take a chance for a split?

Kudzu9....

How do you suggest putting these up as shelves? I want to increase the counter space in my kitchen utilizing these....

What's the construction of the wall? Standard studs and sheetrock?

I'm lucky. My apartment is in a 100 year old barn.... The walls are 3/4" thick bead board. :)

That makes things easier. There are many kinds of nice-looking brackets you can install that don't require the shelves to be screwed to them. You can, of course, go with standards and brackets like this:

Or, something like this:

Or this:

Or many other styles and looks: Shelf brackets

In my experience, if you have weight on the shelves they don't need to be screwed to the brackets. If you want a little more security, you can put double-faced tape between the brackets and the bottoms of the wood shelves. If you feel even that's not secure enough, definitely drill pilot holes before putting any screws in.