Did you hear the news? The Library of Congress has made a ton of its photos available on Flickr. We’re talking vast archive and it looks like Flickr will add other significant collections over time. You can even be a part of it by helping them tag the hundreds of images.
You should know that these images don’t have copyright restrictions. (Here is the official FAQ on rights and use.) It looks like – in most instances – you can use them in your books. Just watch the resolution and sizing to make sure the images print correctly.
Who knows… while you’re looking around you might see your great-grandfather attending a baseball game in 1912. If you do, make sure you tell us about it and include a link to the picture.