I have DSLibris and DSreader on my DSi XL. The DSi XL is the same as the regualr DSi (firmware). The older DS and DS lite can be used as ereaders as well, and actually just about any flash cart works on those models.
The Flash Cart doesn't actually change the firmware on the system, so it is considered to be safe. On other portables, you have to alter the firmware which could potentially harm the system. With the DS systems, you are simple running a program off of a cartridge.
You must use micro sd cards because they need to fit inside the flash cart which is inserted into the DS's regular game slot (called slot 1). The SD card slot (slot 2) can't be used because the system is limited in what can be run from that slot (security for Nintendo).
Setting up a Dstti cart is actually really easy. It is sort of a no frills cart with less features, but it is easy to set up and use.
Basically all you do is:
1. format a micro sd card
2. drag the latest official firmware files onto the card (download and unzip the firmware files from
http://www.happydone.com/download.html)
4. insert the micro sd card into the little slot on the DStti cart
5. turn on the dsi and run the "game"
6. if everything looks good and it boots to the card, you can then shut it down, take out the cart, take out the micro sd card, and add any homebrew programs to it that you want.
7. dslibris can be found here:
http://sourceforge.net/projects/ndslibris/
8. dsreader can be found here:
http://www.ds-xtra.com/DSReader#Down...l_Instructions
I hope that is helpful! There are many wonderful, free homebrew programs for the ds that make the device surprisingly versatile. The unfortunate thing is that many people will abuse the ability of the flash cart and use it for piracy.