Grading coins is the single most important skill for a coin collector to have. Plenty of books will show you pictures of coins in various grades, and ask you to

"This comprehensive and easy-to-use guide provides the official American Numismatic Association grading standards for every United States coin minted since 1793

"The newest edition of the universal coin-grading encyclopedia by James F. Ruddy showcases over 1000 enhanced coin-grading pictures and adjectival descriptions

No subject is more important to the rare-coin collector or investor than grading. Learning to grade the right way can save you hundreds or thousands of dollars.