Her Grace in Disgrace (The Widows of Woburn Place) by Claudia Harbaugh

Her Grace in Disgrace: a Traditional Regency Romance in Kindle and paperback formats.
Reginald Aiken, Duke of Warwick is dead and his young widow is not grieving…until the will is read.
Isobel Kennilworth Aiken, Duchess of Warwick spent 6 years of her young life in a loveless marriage. Now, at the age of 24, Isobel is a widow. As Isobel awaits the reading of her late husband’s last will and testament, she feels no grief, but in fact is quite hopeful. She is eager to start her life anew. But, as the droning of the solicitor’s voice washes over her detailing the bequests to various servants and family members, a shock awaits her. The “other woman” was not his mistress, but his lawfully wedded wife and together they had a son. Six year old Reggie is now the Duke of Warwick, displacing Reginald’s brother Charles.