When using a comparator in subtract mode to detect a signal from a lectern, the outputted signal will not accurately reflect the subtraction of the side signal from the lectern's signal. This only happens when the book in the lectern has an odd total number of pages between 5 and 31 (inclusive) and when the book is flipped to its last page. Since two pages are shown at a time, this means the last page will be on the left with the right page being blank.

Steps to Reproduce:

1. Place a lectern.
2. Place a comparator behind the lectern with its primary input side facing the lectern.
3. Set the comparator to subtract mode.
4. Place 15 pieces of redstone dust in a straight line with one end connected to the output side of the comparator
5. Use a Book and Quill to create a written book that has an odd-numbered page total between 5 and 31 inclusively (ex. 5) as indicated at the top of the book, making sure that the last page (odd) has text in it. Previous pages don't need to have text. To have the total pages show as odd, you can delete the even page to the right of the last odd page by clicking the edit button and then the delete (trash can) button for that page.
6. Sign and Close the book to create the written book.
7. Place the written book on the lectern and flip it to the last page possible. The page on the right should be blank while the page on the left should have text.
8. Input a signal of strength 15 into the side of the comparator by placing one piece of redstone dust directly to the side (right or left when facing the front of the lectern) of the comparator and then placing a redstone torch on the other side of the redstone dust so that it only powers the side input of the comparator.

Observed Results:

Before inputting a signal strength of 15 into the side of the comparator, the signal strength outputted by the comparator is observed to be 15. After inputting a signal strength of 15 into the side of the comparator, the signal strength outputted by the comparator is some value greater than zero. The exact value outputted changes depending on the odd number chosen as the page total of the written book. For example, a book with a total of 5 pages will cause the comparator to output a signal strength of 14 in this setup while a book with a total of 11 pages will cause the comparator to output a signal strength of 3 in this setup.

Expected Results:

After inputting a signal strength of 15 into the side of the comparator, the signal strength outputted by the comparator should be zero. This is because, before inputting any signal into the side of the comparator, the comparator outputs a signal strength of 15, and when inputting another signal strength of 15 into the side of the comparator, the resulting output should be the difference between the two signal strengths which would be zero (15-15 = 0).

Notes:

The three attached images show the book that was placed on the lectern and the page that it was set to as well as the output signal strength of the comparator before and after a signal strength of 15 was put into the side of the comparator.

I tested this setup with books with every possible odd number of total pages (between 1 and 50) and only odd numbers between 5 and 31 had this issue. It was also observed that as more pages were added to the book, the lower the signal strength became, eventually becoming zero once it passed 31 total pages.