I have an older MacBook Pro (2008 silver key version) and am running OS X 10.5.8. Library books work fine with the Mac. My local library has a great selection, and I borrow quite frequently from them!
One caution: the Sony ereaders don't work with the Adobe Digital Edition software. ADE for Mac installs fine on your Mac, but your Sony ereader won't show up as a device to transfer to. As of January, there are no plans to fix this issue. No problem though. You just need to install the Sony Reader Library software on your Mac. You can open the ACSM file with the Sony software and transfer it to your Sony ereader with the Sony software just fine. That's what I do! The process is the same for ACSM files from your e-library or from other e-bookstores like Kobo, Borders, Booksonboard, etc.