Using the default settings, are people outside of my home network able to access my files in the public folder?
not unless you have access from the internet into your network, which is highly unlikely.
hacker --> internet --> dsl modem --> linksys router --> My Book World
the router in this scenario would employ NAT which would essentially shield your internal network from the Internet. As long as you’re not configuring inbound services, you’re fine.
also, be sure your wireless network is well protected to prevent unauthorized users from getting to your local network because that would give them clear access to your NAS.
It mainly depends upon your network topology. Any firewoall ? Your DSL is actually playing role of a firerewall ?
Basically, if you don’t have any firewall, then the service is accessible from Internet.
It’s in my opinion highly not recommanded to do so.