This is an email of suggestions I received.  Keeping it here so I remember:

A little more information concerning the C standard library would be really
usefull.  I (sort of) use assembly and C together as prongs of the same fork
and any information on the use of the 2 together is useful to anyone using an
assembler in the real world.

Many folk have their introduction to assembly using the inline assembler
supplied with most C compilers.  A chapter on the asm keyword would be more
than useful (a bit like the previous point but in reverse).

Any information on using assembly on other architectures would also be useful
(I've had this old sparcstation for over a year now and I've never been able
to suss out how the system calls work).  A chapter on RISC (manual
manipulation of the stack and register oriented CPUs etc) could not only help
your readers but make a significant expansion to your audience (for example,
there are a lot of commercial playstation programmers out there who need to
use an assembler but don't know where to start).

I know it sucks, but a lot of programmers use (or have to use) windows.  Maybe
a chapter or two on win32 would increase your audience?  Maybe looking beyond
Linux is an idea.  How about a chapter on other flavours of Unix?  FreeBSD
stack based system calls are interesting and differ from Linux in quite a
significant way.  Why not mention them?