The original white covered "Inside Macintosh" volumes are pretty good with at describing the hardware and how to use SCSI with Macs. SCSI is not for the faint of heart and you do need the real SCSI documentation regarding command formats.
Of course, to keep things Macintosh, when using the Mac hardware, use the provided drivers, don't dive into programming the SCC chip yourself or whatever. By that I mean if you get into the hardware yourself you won't work when running on a IIfx, under A/UX or Basilisk.