I haven't had the opportunity to read that book yet, but I'll get it via ILL at my university library - thanks for the suggestion!
In the context of the 1830s, 1840s, 1850s, and 1860s sources I've been mostly digging through, it seems like "felling" and "hemming" are both used in the context of dressmaking, sometimes interchangeably , sometimes with an author favoring one term more than the other. Largely it seems to differ by use - "hemming" is more commonly used when referring to, well, hems!
It may be that in tailoring, "felling" is more commonly used than hemming, but my knowledge of tailoring is fairly limited, so I have no idea! There are so many manuals on archive.org that are text-searchable though.... I may have to investigate!