Jo Ann - I have three books that talk at some length about filigree
Om Filigran - Filigree, its Technique & History, Filigree in
Norwegian Ownership. Ved Thale Riioen and Alf Boe, Oslo University
Press, 1959. In Norwegian with an English Summary.
Filigran aus Cortina D'Ampezzo um die Jahrhundertwende, by Ruth &
Max Frohlich. Privately Published, Zurich, 1980 - English summary.
B&W pictures of outrageous filigree objects!
Birka V. The Filigree & Granulation Work of the Viking Period by
Wladyslaw Duczko. Almquest & Wiksell International, Stockholm, 1985.
All three were available online just now as I did a search.
Other than these, I have had to look through more general books to
find examples of filigree. Not hard to find as it has been done so
widely and for so long.