The Enlightenment series is set in 1820s Britain. The first three books are best read in order as they chart the love story of David Lauriston and Lord Murdo Balfour. Books 4 and 5 feature separate pairings in the same world and can be read as standalone novels. The Enlightenment books feature themes of self-discovery.
There are two free stories in the Enlightenment series which are best read after reading books 1-3 as they feature spoilers for those books.
Chronologically, Seasons Pass is set between books 1 and 2 and The Bequest is set after book 4.