Explanation of "symbols" and "symbol tables" in ELF files
and at runtime:
http://blogs.oracle.com/ali/entry/inside_elf_symbol_tables

Comprehensive review of ELF and similar formats:
John Levine, "Linkers and Loaders". On the web:
http://www.iecc.com/linker/

Excellent walkthough of dynamic linking from the exploitation
point of view: "Cheating the ELF", by grugq
http://althing.cs.dartmouth.edu/local/subversiveld.pdf

Brilliant work of re-using standard ELF relocation to do
something else, illuminating its computational capabilities
in the process: "Locreate: An Anagram for Relocate", by skape
http://www.uninformed.org/?a=3&t=sumry&v=6

Nifty expalanation of the minimal required features of ELF:
http://www.muppetlabs.com/~breadbox/software/tiny/teensy.html

More links:

http://blogs.oracle.com/ali/entry/gnu_hash_elf_sections
http://blogs.oracle.com/ali/entry/what_are_tentative_symbols