I know how confusing it is.
I usually get books :
Sony store (lrx format-DRM)
from here in lrf or ePub format (non DRM)
feedbooks (non DRM) in ePub and a custom pdf I made (90 x 120 14 pt)
smashwords in Sony lrf or ePub
That is about the extent of what I normally do. Some MR authors have their own sites and I usually buy the lrf or ePub there also.
My order of preference is: lrx, lrf, feedbooks custom pdf, then ePub.
Many of the classic books are much better formatted by the members here on MR. (Some very dedicated members really take the time to make sure they are very nice.) Then try feedbooks. Gutenberg is where I go last to find a book because of the uncertain formatting.