Congratulations on your new 300 and welcome to MR, thecyberphotog
I have a Sony 505 and don't have any experience with the "Kindle only" ebooks.
My basic understanding is that you can not just buy a Kindle ebook and then read it on your Sony.
There is a way around this IF you are willing and have tech experience enough to run scripts and some other programs. It may or may not be illegal to do this, depending on where you live. (More experienced MR members: If this is not correct, please correct.)
The MR Wiki has a lot of information about formats and ebook stores.
One of the advantages of the Sony Reader is that there are many ebook stores that carry formats compatible for it.
Let us know what you like to read and we can suggest some ebook store sites to explore.
Have you explored
feedbooks (free ebooks) and
smashwords (free and for purchase) yet?
I have
a list of MobileRead (MR) Authors on Feedbooks that you may want to check. These MR authors have very generously put their work on feedbooks for free.
My signature lists some MR author members you may want to check out. I have enjoyed quite a few of their books.
Have you checked out all the
free ebooks right here on MR in ePub and those
in Sony format (lrf) that have been beautifully handformatted by some very dedicated MR members?