"Google Books" does not index self-published books.

Established publishers are unlikely to accept a proposal for a self-help or personal finance book unless the author is already famous.

This could explain the rapid decline in this chart since A.D. 2000 as self-publishing has become more feasible/preferred in the internet age.

If you search "finance", "portfolio", or "stock" the chart also takes a nosedive after 2000 AD. There is no shortage of finance books in the age of RobinHood. They just aren't distributed by publishing houses anymore.