Beyond 2NF, there is third normal form (3NF), Boyce-Codd Normal Form (BCNF), as well as 4NF and 5NF - they become increasingly complex with each iteration, and the end result is usually not overly different from 2NF. This book is about PHP, so there is not room to go in any more detail, however there are dozens of books on database theory that would help you out if you wanted to learn more.
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