IMO dslibris is the best as it supports XHTML IIRC formatting while all of the others merely support text -> more or less unformatted. I'm not sure if dslibris will follow links between files or not which reduces it's usefulness if what you have came in multiple parts and you don't know how to merge them into a single file.
Tidy is useful for converting generic HTML into the variant used by dslibris.
DSOrganize and some other apps, e.g. okiwi (we browser) will open up plain HTML formatted files and could serve as a book reader, but would not have the 2 page view supported by dslibris. Also DSOrganize will NOT follow links to other files, not sure about okiwi as that functionality for okiwi was just recently released, support for file://.
Third option: dslinux then a text viewer or links under that. Drawbacks tiny font, and dslinux does run properly on all flash cart(mainly the SCDS1 AFAIK ATM). Links is a pretty complete text based(well most, and effectively under dslinux) web client and will follow links anywhere. Again you won't get your two page view.
Bottom line: IMO reading text on the DS blows hairy donkey balls. Color screens just never cut it for text viewing. Buy an old monochrome palm, monochrom handera palm license, Gemstar e-book reader(see fictionwise), or a Kindle from Amazon if you're serious about reading alot on handhelds. Monochrome screens are INCREDIBLY important if you EVER think that you will use it as a reading device outside in bright sunlight. Color screens are almost completely unuseable in such conditions.
I use my Palm IIIx for reading alot in plucker (plkr.org) format, but the best was the Gemstar (Rocket/Fictionwise now) REB-1100 IIRC mono text reader (sadly dead ATM).
I'd like to get a Kindle, but it's no way near worth c. $400. If they had them for c. $100 (what I paid for the Gemstar) I'd be there in a picosec. (The kindle does actually allow you to put your own stuff onto it but Amazon buries that deeply, as they'd rather have you mail your stuff to them to convert then have to download netting them a nice fee.)
