"[T]he owner of a particular copy or phonorecord lawfully made under this title [...] is entitled, without the authority of the copyright owner, to sell or otherwise dispose of the possession of that copy or phonorecord." - 17 U.S. Code § 109
Also known as the "First Sale Doctrine," section 109 of the copyright code allows the owner of a legal copy of a copyrighted work to display, lend, sell, or even dispose of that copy without permission of the copyright owner. As with all copyright law, there are complexities and specifics within this. For example, section b(1)(A) excludes software and musical works from the doctrine, though it goes on to allow libraries and nonprofit educational institutions to lend musical works and for nonprofit educational institutions to make software available to students, faculty, and staff.
So what does this all mean for you? The First Sale Doctrine is what enables libraries to lend lawful copies of works. So if you have a legal copy of a work, we can make that copy available to others through practices such as course reserves. But it must be a lawfully made copy. You cannot use First Sale Doctrine as an exemption to make additional copies (such as scanning a physical book into a PDF or ripping a CD into digital files). You can use First Sale Doctrine to make a lawful copy available to others in certain ways.