Just wanted to mention that I've updated the wiki page for reading ebooks on the DS after finally getting around to setting up my daughter's to read books. There is one tricky bit, finding the right software to match the homebrew cartridge, that I've provided some helpful links for. Update is here:
https://wiki.mobileread.com/wiki/Nintendo_DS
I went with DSlibris for the ePub support and the portrait, book-like orientation.