Great question, Kellie!
That's a tough one, because there are so many good books out there.
I think as far as general getting-started books go,
Virtual Assistant - The Series by Diana Ennen and Kelly Poelker was overall most helpful, but
The 2-Second Commute by Christine Durst and Michael Haaren was a very close second. Both of these are printed books that have been great resources for their own reasons.
For marketing info, I learned a lot from
The Virtual Assistant's Guide to Marketing by Michelle Jamison and
The Virtual Assistant's Guide to Landing 12 Clients in 12 Months (an e-book) by our very own Doreen Patrick.

All of the above have been reviewed on this site, so feel free to check out the reviews for more info on why they're so great!
I'm also right in the middle of
Book Yourself Solid by Michael Port, and so far it's been an amazing resource! It's really helped me define my business. Be watching for a full review later this month.
So there you go, my top
five business books. I couldn't choose only three because each of these offers their own unique input and information. I'm also a book junkie, so if it were up to me, I'd tell you to read as many books as possible, but unfortunately that doesn't always fit into start-up budgets. Believe me, I understand that!
I'm excited to see others' top business reads. Maybe I'll get some ideas for new books to add to my own library!

Thanks again for bringing this topic up, Kellie!