Well, I suppose I already have to use Cesium for downloaded music, Player Xtreme for already synced videos; now I'll just have to find a dedicated audiobook player for already downloaded audiobooks.
The maker of Cesium, also has "GreenLit" - a dedicated audiobook player, which accesses your system audiobook library, the same way Cesium accesses your system music library.
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I will add, however, that Apple treats authors much better than Amazon. It pays them 70% of retail at all prices and charges no download fee. Amazon claims to pay 70%, but only for ebooks priced from $2.99 to $9.99, and a grossly inflated download fee makes the real royalties more like 60%. Outside that range, Amazon pays only 35%, the worst in the industry.
Last time I looked, the download fee per sale on one of my graphic novels, which sell for ~US$4, would be over US$20 on Amazon's service, if I wanted the 70% revenue cut Apple gives me.