Effort: The Great ROI
UPDATE: Thanks to @JakeWHayes for his vigilant proof-reading wizardry...
Investing in a sales book, sales group, or sales class can mask personal inadequacies around putting in the necessary effort to be successful. There is a difference between investing in "tools" and investing in your vision.
Don't confuse "spending money" with developing discipline, working tirelessly, and fixing problems with your sales skills... It's much harder to be better (but much more rewarding)!
UPDATE: Just read the following quote and had to add it to my post: "My grandfather once told me that there are two kinds of people: those who work and those who take the credit. He told me to try to be in the first group; there was less competition there." Gandhi (1917 - 1984)