I guess I presumed too much. There's a lot about running a kickstarter that isn't completely obvious until maybe you're in the middle of one, or after you've done it and realized you should have done something different.
Not only do you need to account for all costs (printing, shipping, packing materials, everything) but you also need to keep track of your rewards. If you're offering a reward of say a book for everybody who pledges $X, make sure that $X will actually cover the cost of the book, and the cost of packing and shipping that book to the buyer. If you get this wrong things can go haywire pretty quickly and you can lose a chunk of money or worse, accidentally screw over the people who funded you. Oh yeah, and some of your rewards may not be the book you're funding - keep track of those costs too! If (for example) you're printing and mailing bookmarks for people who pledged $2, can you do that? Can you afford to do that? Or are you losing money? Do the math on everything. Do not let anything slip by. Roll the costs into the kickstarter total if you can.
In the comic book circle there was a pretty infamous kickstarter screw up a few years ago where a rather well known writer/artist team kickstarted a book. Even though they exceeded the goal, they hugely underestimated the shipping costs. Rather than just use the excess money from the fundraiser to pay for their mistake to get books to people on time, they started another kickstarter just to fund shipping. Needless to say a lot of people who had already given plenty of money to them were rather unhappy.
The trickiest part here, aside from estimating how many books you need to print in the first place, is deciding how many books you're going to "give away" as "rewards" for the kickstarter. For example, do you print 100 books and aim to sell all 100 books through the kickstarter? Or do you sell 50 through the kickstarter, and hold on to another 50 to sell online or at gallery shows? The conundrum here is all about figuring out how to best market the kickstarter itself rather than the book - are people willing to pledge money if they're not getting the product itself? Do you want to use the kickstarter system to practically allow buyers to preorder? Or do you need funding to print books to sell elsewhere? That's all up to you, but whatever you choose do the math and do it well.
The biggest mistake people make is not accounting for all of their costs before they start.
The second, at least as applies to books, is having way more books printed than one can ever hope to sell. If it's your first go at it, do yourself a favor and go small. Maybe 100 to 200 books, tops. You'll learn all about putting the books together, working with the printer, postage, etc. and you won't get in over your head. Plus, if for whatever reason the books just don't sell after the initial rewards are sent out, you don't end up with a full closet reminding you your work is unmarketable. And if you sell out of books in a month? Just order more. Keep the risk low and keep your sanity.