Wecome defizr!
The link
https://www.mobileread.com/forums/forumdisplay.php?f=126 here at mobileread will give you hundreds of free books formatted for Sony's. They have been formatted by readers in the lrf format. That is the old Sony format, and will work great on your 300.
On my 300 I convert everything to ePub, unless it is PDF and then I leave it alone (unless there is a problem. Any books that I buy, I buy in ePub. Anyway, that's what works for me..........