Summerset Abbey is a series of three full length novels, with one novella (which I have not read) that was published as an after thought to take care of the future of a fourth female character.
I'll just start the review by answering the question: was the series worth reading? The series is set during the Edwardian Era and WWI, and there are a lot of books that compete for space on this bookshelf. It is entertaining, but light weight.
Books that are better than these include, in my opinion, Judith Kinghorn's The Last Summer, Philip Rock's The Passing Bells, Kate Morton's The House at Riverton, Somewhere in France by Jennifer Roberson, and the Maisie Dobbs series by Jacqueline Winspeare, all of which are slightly more weighty than these bits of fluff.
The main issue that I have with the books is that they really don't do a very convincing job of conveying the horror of WWI. The characters seem nearly emotionally untouched by it. I don't know if this is because Ms. Brown just isn't capable of writing scenes that are emotionally resonant, because the characters certainly suffer as a result of the war (one character ends up as an amputee), or if it is because she didn't include any scenes from the war itself.
The books follow the romantic fortunes of three young women: Rowena and Victoria, who are siblings, and their adopted (but not legally adopted) "sister," Prudence, who was the daughter of the maid but was raised like a member of the family by their unconventional father. The series begins with the death of their father, and they are thrown out of their comfort zone to live with their uncle - their father's brother. That side of the family fails to treat Prudence like a member of the family and relegates her to the servant's quarters, separating her from Rowena and Victoria.
On the whole, the books are well-written and enjoyable, if occasionally frustrating. They aren't great literature, but they are a nice historical read that doesn't offend for readers who are looking for something to occupy their time while they wait for the next episode of Downton Abbey.