I’ve owned a (signed!) copy of Andrew Glassner’s Principles of Digital Image Synthesis for a short while. It’s an amazingly in-depth book on the fundamentals of computer graphics, and I warmly recommend it. Anyway, I recently happened to glance at the appendices and noticed Appendix G. This appendix has tons of interesting spectral data in table and graph form: indices of refraction and extinction for various materials, CIE observer and human cone response curves, and spectra of various light sources. The appendix claims that the data is also available via ftp, but unfortunately the directory it gives doesn’t exist anymore. However, the appendix also mentions that the spectral reflectance data given for real objects is a subset of a larger set that is also available on ftp, and that one appears to still exist, albeit at a slightly different address (if your browser has problems with ftp, use a standalone client to open an anonymous ftp connection to ftp.eos.ncsu.edu and go to pub/eos/pub/spectra). This site has reflectance spectra for various color chips including the 64 Munsell colors, as well as 170 objects such as various types of wood, human skin and hair, leaves, rocks, many types of fabric, etc.
A good source for spectral data is handy even if you are not doing spectral rendering; you can convert them into RGB values and use them as reference reflectance values (I did that recently when I needed a good reference RGB reflectance for copper).