The phrase speculate to accumulate could have been written by a publisher since speculation is at the heart of the process; whether it be an agent deciding which author to sign up or a production director deciding how many copies of a book to sign off. Judgment and not hard numbers is what underlines the process of manuscript purchasing and book publishing.
Before a manuscript becomes a book it will have been through myriad judgment calls on the text, the cover, the market and the sales expectations. Ultimately the only real judgment that counts, that one decision that either confirms or undermines what everyone has thought about a book, is that of the reader, the person who buys the book. Ironically, when choosing what to take on, agents or editors are the same as the people they want to reach: readers deciding if they like a story. If they do like the story and they can persuade others to as well, the manuscript has a chance of becoming a book.