Good Question! Here is my hierarchy.

For Cloth-bound Books!

Not for leather or suede or vellum or paper or even plastic.

This is a rough hierarchy of methods to be considered for book cleaning.

1. DRY: Eraser, Knife, Sandpaper. Use an eraser, work in small circles with light pressure. If more is needed you can use something sharper. A micro-spatula can be sharpened and used to scrape particles off cloth or use a book repair knife. Sandpaper can also be useful in some instances. Test an inconspicuous area. All of these methods may remove some of the color. The scraped areas could be re-touched using colored pencils or acrylics but that is an advanced repair. This is definitely something to try on a practice book.
2. DRY-MIX: Isopropyl Alcohol / Water Mix: Simply add isopropyl alcohol to water (60/40 or so)
3. WET: Just Water. Use a humidified cloth. If you squeeze the cloth, no water should drip out.
4. Chemicals: Things like 409 or any other commercial cleaners. If these are wiped onto book-cloth can just get stuck in the fibers and will degrade the cloth over time. The exception is coated cloth that has a plastic element to it. Plastic, of course, can be wiped with your standard kitchen cleaners.