Update 'backlog.md'

backlog.md

@@ -8,6 +8,9 @@
 - 2-Categories
 - Reflective subcategory
 
+## Book Backlog
+- David Allen's Getting Things Done
+
 ## Reading Stack Backlog
 - [Marx, Engels and the Abolition of the Family](https://www.csustan.edu/sites/default/files/History/Faculty/Weikart/Marx-Engels-and-the-Abolition-of-the-Family.pdf) *(added 5/3/2023)*
 - [Cryptography](https://en.wikipedia.org/wiki/Cryptography) *(added 5/3/2023)*