Hi everybody, we've made some good progress and have two quick updates for you all:
Update #1: ROM dumper data corruption mitigated
We finally discovered the reason why the ROM dumper was sometimes getting corrupted data.
The Bookman cards have fairly loose tolerances in their physical manufacture. Every card has its own slight deviations within the plastic shell that holds the circuit board containing the memory chips. Bookman reader devices electrically connect to the card using an elastomeric connector (conductive rubberized strip); the electrical pins of the card have to be levered into this strip in order for the reader to work correctly. 
Returning to the manufacturing deviations: the two tabs at the side of the cards act as a pivot point for the leverage needed. If they are positioned slightly differently that affects how much pressure is loaded onto the elastomeric connector, which means the pins were guaranteed to make full contact. Trying to read from the card meant fetching junk data.
Solution was simple: put a weight onto the top edge of the card while it's plugged into the slot. So far every single card dump using this fix has dumped successfully. We also managed to found the first card in the collection with real data on two memory chips rather than the usual one!
Many thanks to the soup can!
Update #2: Card Icon images reverse engineered
We worked out how to consistently extract Card Icons from the ROM files through some careful binary analysis. This procedure works 90% of the time, and we still have to catch some edge cases for a few ROMs. 
See the investigation notes here: analysis.md
We hope you share our excitement as we continue making progress on the reverse engineering. Please stay tuned!