To begin, choose what book cover you want to recreate.
Begin with the base layer of the book cover. Pick, or blend, wool that matches the base layer.
Colored wool
Take your felt (or wool) base sheet and place it on top of the foam block.
Foam base, Wool sheets
Now, take your base color and place it on the felt base where it's appropriate for your cover.
Stab the colored wool into the base sheet by repeatedly bringing the triple needle in and out of the wool sheet and foam block. Careful not to stab your fingers or go too hard that you go all the way through your foam block.
Continue to add more of your base color and stabbing it in until you are happy with how it looks. Don't be afraid to layer other colors into the base to add dimension or texture as needed.
Every cover will be different, some may have multiple layers with details on top. Follow the same process as the base layer as you continue to add layers to your cover.
Once you have your base layer, or layers, completed, it's time to add details. You'll follow the same process for picking colors and blending for this.
With your colors ready, map out with the wool where you want your details to be.
When ready, stab the details into place using the appropriate needle tool.
Add details until you are satisfied with how it looks.
Remove the final product from the block and put it in a frame to be put on display.
Notes
Don't forget, if you put a strand in that you don't love, you can remove it. So don't be afraid to make mistakes or to try something.
Using the single-needled tool at an angle will help define shapes and make cleaner lines.
The more you stab something in the more integrated it will become into the base sheet. If you want to add depth to your cover, make some layers more matted in than others.
Making small shapes can be tedious; try forming them by hand, then stabbing them into place with the single-needle.
Avoid covers that have small words unless you want to leave them off. Or you can mix mediums and embroider them on later.