I have recently written a Linux (RH 7.1 - 2.4.2-2) driver for the PCI 6527 (as well as a 6527 driver for QNX 4.2 and QNX 6.1 RTP). Most of it was straight forward, except for 2 areas. 1) Mapping the cards into User space is a little tricky. 2) Getting interrupts to work was a little tricky as well. I did eventually get them to work, but it was trial and error. I think the Register-Level programming manual is in error (or it appears to be).
A good spot to start for Linux drivers is the O'Reilly book by Alessandro Rubini called "Linux Device Drivers". ISBN 0-596-00008-1. 2nd Edition released June 2001 covers Linux 2.4. Chapters 3, 7, 8 and 9 are well used in my book.