Welcome to the forum stevet. Be sure and check out all the free books here at mobileread
https://www.mobileread.com/forums/forumdisplay.php?f=126 that are ready to go for your Sony. You can also look a few forums down from that one and get many ePub books that will also work..... And here is a link to free ebooks from the Sony store
http://ebooks.addall.com/sonyfree.html . Be sure to check back on the Sony link as the book content changes. Also Kobo is a great sight for buying books
http://www.kobobooks.com/ and is often cheaper than the Sony site.
Also.... +1 for Calibre
And as Scion said, stop on by the Sony forum!
Again, congrats and have fun!