Do I Need a Book Proposal? (Part 1 of 2)

I used to be the kind of writer who hated with the utmost hatred the idea of writing a book proposal. I may have even whined that my manuscripts should be judged on literary merit rather than marketing potential. I stubbornly refused to query agents or editors who required book proposals (which severely limited my prospects, by the way).
Then one day I got a boomerang response from an agent who