A mic (or “mike”) should be your friend. And like a friend, you learn what works and what doesn’t when you are working and communicating with each other. Mics don’t like to be blown on, don’t like you to get too far away that they can’t hear you, either. Mics can be sensitive or not, your friend or your foe.
Metaphors aside, when you purchase a mic, begin with a mic you can afford, but don’t expect to get anything good under $50. Purchase one at a store (in the D.C. area, try Best Buy before Chuck Levins) so that you can hear what it sounds like (and forgo the shipping $ and time) or purchase online through a retail outlet, or buy on Ebay if you know what you’re doing (and make sure the seller offers returns). Try Craigslist if you’ve a mind to, but make sure you have the seller set it up and run the mic before you buy.
If you can afford it, buy more than one, and get two different mics with different make-ups and sounds. For audiobook work, I recommend picking one that works for you and sticking with it. They (the publisher/studio) will approve the sound of your space (noise floor, etc.) and your mic — that’s usually good enough for everybody. However, making voice-over recordings may require something more, something more of a boss jock radio-announcer-quality, for instance.
A good friend to the local voice-over community: http://www.modernvoice.com/#null