First, let me apologize for the conflicting responses here. That was my doing. I made some sweeping changes to the library while the rest of the crew was very deeply immersed in the office move, and the impact of those changes did not get clearly communicated.
readRegister(x) no longer exists in the latest library -- it was superseded by readID(), which should be doing the exact same thing as readRegister(0x0), but, for whatever reason, isn't. (This change was part of an effort to reduce the code size in order to fit on the Arduino Leonardo...readRegister(0x0) was the only case actually being used, so the function was simplified.)
Until we can get to the bottom of this, I'd suggest one of the following:
1) keep using your modified library (where you're accepting the 0x5555 ID and continuing with the 0x7575 case) and, if we can find the cause, eventually a library update will correctly handle this case.
2) roll back to the older library (the version using readRegister(0x0). Unless you're using an Arduino Leonardo, this should not impact your work; that version is otherwise known working.
If we can't identify the cause, we may end up just doing #2 anyway, and would have to abandon Leonardo support in the library for the time being.