You have inventory. You need to enter in the Cost of Goods Sold section of Schedule C the expense that went into the books (you can list them on 36a as Purchases, assuming you bought the completed books).
Then, for inventory, you will list the value of the inventory you had on hand at the end of the year (100 books). This should be the same value as the Purchases.
This will result in a zero for Cost of Good Sold for 2018. Therefore, you will have no loss reported due to buying the books (you could easily have other expenses, though, that could cause a loss in 2018).
What will happen is that you will recognize the expense (buying the books) in the year that you sell the books (hopefully 2019). This will happen when your inventory shrinks so that you have expenses to apply to your income.
