Sorry for posting in this old thread, but I'm looking for an answer to the same question.
I'm afraid that I've ruled out Stinger's solution as a workaround by taking the following steps:
1. Removing the book, rebooting, adding the book, rebooting does not cause the cover to appear when in 'sleep'.
2. Inspecting the file____mnt_onboard_[...].parsed files for the book reveals that they are valid image files with the correct book cover.
So the problem is not corrupt or missing file____mnt_onboard_[...].parsed files.
Interestingly, I notice that the 'missing' book covers do display under the following circumstances:
a) the book in question was the last book open before the device was turned off
b) The device has gone into sleep mode directly after turning on without loading the book yet.
When this occurs, the 'missing' cover displays normally, even though it stops displaying on sleep mode after the book has been loaded.