Linux-insides: Linkers

I'd also recommend "Linkers and Loaders". It's a bit dated, but it's a classic and most of the core underlying ideas are still relevant today:

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

This is an in-depth introduction to linkers by Ian Lance Taylor, the author of the gold linker. Before reading this I thought I knew what linker did and how it worked, this series set me straight.