These weren't the ebook readers you'd expect today and in fact they have very little in common with most ereaders on the market. The Data Discman were conceived in a pre-PDA, pre-smartphone gadget ecosystem. This was so long ago that it even predates modestly priced laptops. They didn't have many fiction titles available, and they were also rather expensive (prices started around $400 and went up from there).
The DD-S35 was one of the last Data Discman, and it was priced around 44 thousand yen when it was launched 10 years ago. Depending on the conversion rate and inflation, that would be about $600 to $750 today.
The prices limited the market to those who really needed the compact format, and that probably affected the title selection. Most of the ones I'd seen were reference guides, encyclopedias, and other dense information sources.
The Data Discman used a proprietary cartridge that Sony sold but inside the cartridge was a standard 3.5" CD. (No, seriously, you can still buy this CD size if you want.) The file format was also proprietary, and while the format has supposedly been hacked, I never had much success in burning my own CDs for the Data Discman.
BTW, there were actually 3 (or possibly 4) file formats for the Data Discman, not one. The base format only supported text, and the more sophisticated formats had images and audio.
The multiple formats made things a little interesting for end user, because the several dozen Data Discman models didn't all support images and audio. The various models also came in a variety of shapes, including slabs and clamshell.
I just got a DD-S35 off of Ebay. It's not the sturdiest of the models, but it's also one of the few I have that is completely functional. It has a keyboard, a 5" LCD screen (320x240), speaker, headphone jack, and it runs off 2 AA batteries. My unit was originally released in Japan, so I'm at something of a loss to figure out what the symbols on the keyboard mean. On the upside, the DD-S35 does support the enhanced formats. At least it should; I haven't found any images or audio to test.
The software is rather limited, but the hardware is quite well done. That's no surprise; according to one source I have one of the last Data Discman model that Sony ever released. It hit the market in 2000.