My humble suggestion: "How to Write Shared Libraries" by Ulrich Drepper

http://www.akkadia.org/drepper/dsohowto.pdf

Is this instead of or in addition to the original article? If the latter, in what order ought they be read?

I would suggest that you read the beginner's guide first, then John Levine's Linkers and Loaders book, and how to write shared libraries. The last one is really comprehensive and can seem complex at first. You might want to see big picture first :)

John Levine's Linkers and Loaders book is free online :-)

http://www.iecc.com/linker/

How timely, just as I've decided to write a shared library for Varnish.

Thanks muchly.