Particularly useful sources of information about libraries include the following:
``The GCC HOWTO'' by Daniel Barlow. In particular, this HOWTO discusses compiler options for creating libraries and how to query libraries. It covers information not covered here, and vice versa. This HOWTO is available through the Linux Documentation Project at http://www.linuxdoc.org.
``Executable and Linkable Format (ELF)'' by the Tool Interface Standards (TIS) committee (this is actually one chapter of the Portable Formats Specification Version 1.1 by the same committee). This provides information about the ELF format (it isn't specific to Linux or GNU gcc), and provides a great deal of detail on the ELF format. See ftp://tsx-11.mit.edu/pub/linux/packages/GCC/ELF.doc.tar.gz If you get the file from MIT, note that the format is unusual; after gunzipping and untarring, you'll get an ``hps'' file; just strip off the top and bottom lines, rename it to a ``ps'' file, and you'll get a printable Postscript file with the usual filename.
``ELF: From the Programmer's Perspective'' by Hongjui Lu. This gives Linux and GNU gcc-specific information on ELF, and is available at ftp://tsx-11.mit.edu/pub/linux/packages/GCC/elf.ps.gz.