I think you yourself see the problem with the answer, as you prelude it with:
If you have access to it, (...)
As I understand, content in MSE should be as self-contained as possible. This "policy" is one of the reasons why images are deprecated, for example. The fact that it is "one of the major works on the history of notation" is irrelevant, in my opinion. In a few years, it may be less widespread, copies may be lacking. Also, acessibility depends on where you are etc. For instance, I've just made a search on the author in the database of UFRJ (Universidade Federal do Rio de Janeiro) for books on every library in all campi, and made hits for
- An introduction to the modern theory of equations.
- Sir Isaac Newton’s mathematical principles of natural philosophy and his system of the world
- A history of elementairy [sic] mathematics with hints on methods of teaching
- Storia della fisica elementare con l’evoluzione dei laboratori fisici
- A history of mathematics
The book you mention is not on the list. This illustrates the problem nicely, I believe.
That said, your answer may be good (none of the above says otherwise. It only argues that it may be inacessible now for some, or that its acessibility may change with time, which are not good things for one of the goals of MSE of a self-contained body of knowledge I believe). But if you interpreted that "the question asks for a history lesson on the history of the notation for derivatives, Cajori wrote half a tome on precisely that", then the problem is with the question being too broad. The description of the "too broad" close vote changed, but previously it had a fragment which said:
(...) good answers would be too long for this format. (...)
It seems that it was changed partly because "too long" is not very precise and may seem to frown upon long but objective answers (more information here). But certainly, if you think that referring a book for a person to read about is the best answer in this platform, I think that you consider the question to be too broad (and I agree with this. Were it not for the fact that the question is 8 years old, I would vote to close it).