The Bloser book is excellent - indispensable in fact - for Ornamental Penmanship, but for textbook Spencerian, you really could do with a textbook by the Spencer brothers.
The one I'd recommend is
The Theory of Spencerian Penmanship, simply because there is a printed edition available from Mott Media at a very reasonable price, which is well worth buying. As a complement to that,
The Payson, Dunton, & Scribner manual of penmanship is rather more informative about certain practical matters. There are also the two Spencerian Compendia,
old and
new, which you should certainly have a look at, but there are considerable differences between the two which could lead to confusion. Most people will prefer the latter, I should think.
My take on Spencerian is a fundamentalist one, and I see no reason why anyone should look beyond the original 19th century exemplars. Rather unusually, the style is carved in stone, as it were. There are some very poor alternative exemplars out there, and I'd suggest that you have nothing to do with them. It's different for Ornamental Penmanship though (an elaborated form of Spencerian), and there are many excellent resources to browse, starting with the rare books section at
IAMPETH.
