Apple II GCR format information needed!!

[adq@tardis.ed.ac.uk (Andrew De Quincey)]

Hi, I'm writing an Apple ][ GCR disk decoder as I speak....
I need some information on the disk format....

I'm able to read an Apple ][ GCR disk in, find the sectors, and
extract the sector information. My problem comes with the actual
data part of the sector. I don't know exactly how to decode the 
6:2 (or 5:3) encoding of the sector data...

I know it involves a lookup table, which I have. My problem is
reassembling the 8-bit bytes from the 342 6-bit nibbles.. I don't
know which bits are part of which byte...

Also, how are the sector data and sector ID checksums computed...
I think the Sector header checksum is a simple EOR of all the bytes
in the header.... at least this seems to work....

I also have no information at all on the older DOS 3.2.1 5:3 
encoded disks... can anyone help me with this???

I have one document describing the 6:2 encoding, and the sector
checksum, but it has several errors in it, and is not precise
enough on *exactly* how the sector data/checksums is encoded....

I know that the book "Beneath Apple DOS" describes all this, but I
do not have a copy, and am unable find an old copy of it.

Please note that I am unable to use any calls in Apple machines,
because this software does not run on Apple hardware...

Thanks....

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adq