Plot in Persuasion

Like many people of the upper class in rural England, Baronet Eliot cannot afford to maintain his lifestyle, and so he goes into a death spiral, hoping to get an influx of cash from a marriage or some other source.

The phrase "book of books" refers to Debrett's Baronetage, where the history of each member of the family is recorded. The book is a painful reminder to Elizabeth that she is still unmarried.

At age 29, Elizabeth will soon become less attractive as a marriage candidate.  Most women of Elizabeth's stature were married by their early 20s.

In other words, Elizabeth is the only daughter who can be expected to marry someone of her own family's status.

It is unusual that neither should marry. Men with property and a title were expected to marry if a suitable partner was available, and widows were vulnerable socially and economically.

In other words, the fate of Lady Eliot's three daughters is too important to put in the hands of her "silly" husband.