A larger impression pool is no help if the impressions are being seen by people who will not buy what you are selling. It took me dozens of tests to find a good list of comparable authors but your bids also have to be right. Frankly for testing, I don't suggest bidding low. Instead, you might use a low budget, but bid average. For each test, I set about a $15 budget so that I don't overspend but hit the average bid that BB suggests. That might give you a better look at how the authors perform.
On the subject of Also Boughts, sometimes it can be worthwhile to dig a bit deeper than your own list. If you share an audience with one particular author, you might go to that author's page and see what authors he has on his page. If they aren't already on your list, I'd suggest testing them. Then be sure that they have followers on BB. There is no point in targeting authors who have very few BB followers.
