I've been on both sides. I've worked in libraries where we had .15/day fines, capped at $5/item, but NOT capped for # of items, so I've seen accounts with 100s in fines, even $750, because after a VERY long time (over a year, I think) the library considers an item lost/stolen and replacement cost is charged to the account. $25 + $5 administrative fee x 20 or 30 books = Big fine.
I have also paid more in fines than an item was worth (paperback = $3.99 cover price, $5 max fine... )
Consider that many home organization shows say "Just toss all your old books, after all the library has them all!" and then books which are out-of-print (which can happen very quickly, only a couple of years sometimes) are only available at the library. Oops, you want it but can't buy it new? Just say you "lost" it, and keep it for the replacement cost! If libraries charged the going rate on eBay, abebooks.com, or other rare book search sites for some of their "lost" items, there would be a much higher return rate of lost items by patrons. There is a huge problem at libraries losing rare books that nobody (except the thief) knew were valuable. Now only one person has access to it, and the library can't replace it, as it's out-of-print.
Can't you check and see if you can get it used from Amazon or B&N's used section? I've had really good luck on there before. Then, you have the book, for a reduced price, with a degree of security from the online bookseller backing, and no more guilt!
Better yet, try your LYS, often they sell used knitting books. It wouldn't be cheap, but you would be supporting your LYS and keeping them in business to feed your yarn habit.