Question: is it illegal to be paid off the books

Answer: Yes, Unreported employment; working under the table, off the books, cash-in-hand, illicit work or (in UK English) moonlighting, is illegal employment that is not reported to the government. The employer or the employee often does so for tax evasion or avoiding or violating other laws. The working contract is made without social security costs, and does typically not provide health insurance, paid parental leave, paid vacation or pension funds. It is a part of what has been called the underground economy, shadow economy, black market or the non-observed economy.