Can royalties be garnished in an effort to satisfy a debt even if that is your only source of income?

Royalty income is not exempt from garnishment, the way certain other kinds of income are exempt: for example, social security income, most pension/retirement income, and some payments from insurance are exempted from garnishment. The fact that this is your only source of income does not offer any protection; after all, wages and salary can be garnished (in most wages; there are a few states, like TX or NC, which do not allow wage garnishment) even though for most people, wage or salary is their only source of income.

Depending on how much you owe, you may wish to consider bankruptcy as an option.