I have done several days of additional research on this and found out that it appears I can deduct the cost of the books against a single year's royalty income by claiming a Section 179 deduction. The steps are as follows:
(1) Write the maximum amount of property you can claim under section 179 on line 1 of Form 4562.
(2) Add up the total cost of section 179 property you began using during the tax year, including books, and record the amount on line 2.
(3) Write the limit of your deductions on line 3.
(4) Subtract the amount on line 2 from the amount on line 3 and record it on line 4. If line 3 is larger than line 2, simply write "0" on line 4, then subtract the amount on line 4 from the amount on line 1 and record on line 5.
(5) Describe the property and books on line 6 and record the cost of each in section b. Write the amount of the expense you are claiming for each item in section c of line 6. You can claim the entire cost of the books.
(6) Add the amount in line 6 c to any amounts on line 7 and write the total on line 8. Write either the amount on line 8 or the amount on 5 on line 9, depending on which is smaller.
(7) Write the amount of your Schedule C income on line 11, unless it is greater than $500,000. On line 12, write the amount of your deduction, which is the total of line 9 plus any carry-over you may have had from the previous year.
(8) Record the amount of your deduction for section 179 books and property on line 13 of your Schedule C, not line 22. Include form 4562 when you hand in your tax return.
source: "How to Deduct Books for Self-Employed" by Emily Weller