The second book in the series, The Viscount Who Loved Me, focuses on Anthony resolving to marry a woman suitable to be his Viscountess and falling in love with her sister instead.
It is not, however, interesting enough to sustain an entire season. Especially not when the show has gone to great lengths to set the plot in motion for many of the characters who do not receive story arcs until later books.
In all honesty, I initially thought the show was taking things in a wildly different direction with Benedict.
While neither of these books are enough to make a full season on their own without rehashing things we’ve seen before, combining them into one season would actually illustrate how similar the two brothers are, despite appearances to the contrary.