The best way to get people motivated about helping the library is to show off all the great things the library does, and to demonstrate the things that could be better if only the library had better funding. It's a great idea, therefore, to host an open day or other event where people are invited to come see the library and everything it has to offer, and hopefully while they're there they'll be generous enough to help your public library fundraising out in some small way.
The trick, of course, is getting people into the library in the first place. Food usually works - if you have a place where you can set up food and drink booths (maybe outside if you're worried about having food in the building itself), your fundraiser can be a lot more enticing, and you can make a decent amount of money that way.
If your building is large or historical, you could even charge a small amount for guided tours, teaching the history of the building and showing everything it has to offer. Once people can see what the library is all about, they'll be much more likely to give to your library fundraiser. You can also set up some supplementary activities that will help bring in even more money. Scratch cards are very easy to hand out at an event, and they bring in very high profits, so with just that little bit of extra effort, your fundraising effort can go from a moderate level of success to amounts of profit you didn't think were possible.