Rich Whitehouse has released a text viewer for the Nintendo DS, heres what he says:

This is nothing special, just a text viewer for the DS. Makes use of a full touchscreen interface, persistent text bookmarking through SRAM, low power mode, and multiple file support with clean GUI for selecting. Uses a 5-pixel-width subpixel font to allow greater text density than standard 8-pixel tile fonts.

Download here --> http://nintendo-ds.dcemu.co.uk/textviewer.shtml