I've found LDD to be an invaluable resource for programming drivers on an x86.
However, there is a great big PowerPC shaped hole in the book. There are a great deal of areas in doing device drivers for the PPC that, as far as I can tell, are not documented ANYWHERE (including the source, which as a paucity of comments, making it less than useful as a reference guide).
I cannot help write the section (because if I understood it well enough to write it, I wouldn't need the section!), but I can tell you some areas that I think could use a good write-up:
0) General embedded PPC type issues - working with some of the Freescale embedded devices, for example.
1) OpenFirmware and the OF device tree - what it is, who sets it up, how to extend it for your own devices.
2) MPIC interrupts, and how to get from the MPIC hardware to request_irq().
3) Memory space management: how to map devices that occupy address space into RAM when they aren't bound to some PCI device.
I also think some more general topics need to be covered better:
1) Memory space management: how to map devices that occupy address space into RAM when they aren't bound to some PCI device. (yes, this is a repeat of #3 above, but this is more generic).
2) Allocating large blocks of memory aligned by size (e.g. how to allocate a 64K block that is aligned on 64K) - this comes up with a number of DMA devices and address space translators (like PCI interfaces).
3) Dealing with PCI devices that can change their configuration at run time (e.g. FPGAs that, upon being programmed, export new BARS). A set of "best practices" for dealing with such devices ("should they generate hot plug events? if so, how?") would be invaluable to people creating such hardware.