Book GUI is not vertically centered

The Bug:

The book Gui displays at the top of the screen. This is kinda annoying and seems like a bug. This happends since the Writing Book gets added.

Steps to Reproduce:

1. Obtain a book and quill.
2. Set your GUI scale to a small value, for example, "1".
3. Open the book and quill.
4. Take note as to whether or not the book GUI is vertically centered.

Observed Behavior:

The book GUI isn't vertically centered.

Expected Behavior:

The book GUI would be vertically centered.

Code Analysis:

Code analysis and potential fix by notmattw can be found in this comment.