Depending on the size of your book and the size of the groves you want to add there are different tools you can use:
You can make the natural grain of the wood more defined by running a wire brush along the surface several times. Always drag the brush from one corner to the other one to avoid unevenes in the structure. Wire brushes come in different hardneses and the harder your brush is, the more defined the structure becomes. I only ever used a hand wire brush for this technique and I suspect a rotary tool doesn't work well.
Test on a waste piece how the wood reacts to this treatment. In general this creates a very fine texture like natural wood grain. If there are areas of softer and harder wood, the texture will be uneven.
You could simply saw some very shallow lines into the wood with a hacksaw. I'm thinking about 1mm depth, so it's basically just "drawing a line" instead of actually sawing.
The disadvantage is that you might create a lot of splinters and it's hard to create an even line over the whole length of wood. Mos of the time the cut is deeper at the edges than in the middle of the wood.
X-acto knife / crafting scalpel
You can create fine lines by dragging the tip over the wood. In the first pass, slant the knife to one side and in the second pass slant it to the other side. The result is a narrow V-shaped groove. You can see a very short tutorial on YouTube.
The advantage of this method is that you can create thick and thin lines and either very exact and parallel lines or lightly uneven lines with the same tool, depending on how much you slant it and how straight you cut the lines.
Nail or other sharp object
You can drag the tip of a nail or similar sharp point over the wood. The advantage of this method is that you can use a ruler to make the lines straight. It also creates a rougher texture than the wire brush, which might suit a big object better.
Apart from a nail (that might need sharpening) you can probably also use the tip of a screw, the edge of a hacksaw blade or metal ruler or other hard edges you find in your workshop. Again, try it on a waste piece of wood to see how your individual tool works.
You may already have one lying around, but it may not be long enough for your book. Just file some shallow grooves into the wood. Pretty self-explanatory.
Whatever you can find lying around
I'm sure there are countless other methods you could use. Look around at what objects you already have and experiment.