soulfuldog and pepe_alter_ego,
Thank you both very much! I appreciate your suggestions.
@soulfuldog,
I have read through several discussion threads here about AZW vs. .mobi, and it appears that the general consensus is AZW is the preferred conversion format in this case. So you've confirmed that I haven't completely misunderstood the situation.
@pepe_alter_ego,
Until you mentioned the possibility, I wasn't even aware that I could "buy" PG eBooks for free from the Kindle Store. I think my wife might like having access to some of those extra features like X-Ray and such. It's definitely worth investigating.
(I am making progress convincing her that a front-lit e-reader would be enjoyable / useful. However, she's a die-hard fan of paper books, so I'm not done yet ... I think my next tactic might be to remind her that our eyes are aging and needing larger fonts).