Self-help, in the legal context, refers to obtaining relief or enforcing one's rights without going through legal processes, such as repossessing a car when payments are past due, retrieving borrowed or stolen goods, or demanding and receiving payment. Self-help is legal as long as it does not "disturb the peace" or violate some other law.
Self-help legal books are written for people who aren't lawyers. They explain the law in plain English and sometimes give you tips on how to handle your case. Many law libraries and public libraries can loan you self-help books.