I'd heard of it, but I have a long-time interest in very specialized aspects of education.
You've gotten marketing advice (add bit: if you have free ebooks online, hand out cards with the URL), so I'll throw around some advice about topics.
Almost anything that's regularly discussed here at MR is good. For a library, anything about digital archives and preservation of texts is more than appropriate: the
Hathi Trust lawsuit,
library ebook costs, whether libraries should or should not carry indie/self-pub books, digital preservation of rare/fragile books, etc.
Or anything about writing is appropriate... how to actually *finish* writing a novel (there's no special trick to this, but the public doesn't get tired of hearing it as long as they don't feel like they're being insulted for not having done so), differences between academic and commercial writing, author styles--outlines vs "just start writing" vs writing some scenes and later assembling them, etc.
If you know the history of the library, that's usable, but what's usually mentioned in "history of this building/organization" is dull. If there was a scandal or a passion behind the library's creation--"the founder grew up without a library nearby and swore that students would always have something to read, which is why the library keeps longer hours than most bookstores" (or whatever), that's worth talking about. If the library has interesting programs or special collections, you can talk about those.
Don't try to figure out The Best Topic For A Library Speaker; don't assume you need to do "Why I Love Reading And Why Libraries Are Awesome." The people attending already know that libraries are awesome, and they also love reading; that's not an interesting topic unless your particular take on the awesomeness of libraries is unique. ("Libraries opened new worlds to me" is not unique. "My friends built a code around the Dewey Decimal System and arranged secret meetings by mentioning book titles when our parents were listening" is unique.)
Assume they asked you because they like *you*; find a topic you're happy to discuss with a few dozen strangers and wouldn't mind a few thousand (the readers of the local papers) knowing what you think about it. Assume you have between ten minutes and half an hour; if you need more specific info about that, contact them.