1. I have an 8" unit WITH a CD player (DVD actually), after-market, repalced the stock 5" unit (that came with a CD too). Except Sirius, all OEM functions are working fine. Most aftermarket units are for some reason not compatible with the higher-end Tucsons though - I think a forum member navein was trying to figure out how to hook up his after-market to the OEM preamp.
2. In case you want to go the "add-on" route, of course, to get the benefit of higher fidelity sound of CD you want go connect to AUX IN. But, for audio books, I would just pair a "vintage" CD player (I think I have one in a drawer with a CL adapter), and a FM transmitter. This way, all you need is to run a power cable from the "CL" power port to let's say the glovebox, or under the seat, and keep a player and FM transmitter there. Personally, I would rather transfer to USB though.
3. The unit that I have actually has secondary microSD card slot (primary is for navigation). It easily accepted 32GB memory card and plays mp3s a wmas (haven't tried other formats). It also should support saving (ripping) CDs to this memory card - there's a button on the interface, but haven't tested it yet.
4. I have been long frustrated with Hyundai's corporate approach to head units. Even in Tucsons, there are several options (based on trim and market), mutually incompatible with no option for upgrade, and they are making every effort for using standard aftermarket solutions very difficult.I love our new Tucson - and I am very happy with aftermarket unit I found and installed - but not at all impressed I had to do so in the first place.