Hi Mike,
The Victorian Scrollwork is a good start. My 3 other favorites (for now) are French Baroque Ornament, Baroque Ornament and Designs by Jacques Stella, and Florid Victorian Ornament by Klimsch. Keep in mind that I am currently focusing on the 16th and 17th centuries, hence the Baroque designs. However, the books are less useful for using as examples of engraving for guns and much more valuable for learning the styles of work at the time and the actual techniques used for shading and highlighting. Another book to have if you can find it is Grancsay's Master Gunsmiths' Designs. This book was published in 1950 by the Metropolitan Museum of Art in New York. The quality of the illustrations is beyond compare, particularly when you look at them under magnification. They will teach you a tremendous amount about shading and varying styles of cuts. All of those sources show engravings far more technically advanced and sophisticated than anything found on American longrifles. However, they set the bar high and if you can even partially approach that level of work, engraving a longrifle will be a snap. You'll be able to do it in your sleep.
dave