You really need to have read the Outlander series to understand the context of what is going on here. 

vpreading's review

3.5
adventurous mysterious tense medium-paced
Plot or Character Driven: Plot
Strong character development: No
Loveable characters: Complicated
Diverse cast of characters: No
Flaws of characters a main focus: Yes

This book was a solid 3-star book until the last quarter when all the pieces came together. Even though Lord John is my favorite character in the Outlander series, I was still hesitant to read his stories. I do think the book was a little slow, and since I'm not big on mysteries, I knew I wouldn't be drawn in right away. This book was more lighthearted than I imagined, and I am going to read the next one.

I like the storyline and the amazing way she writes.

This book was too slow moving for me. I'm a huge Outlander fan, but the Lord John series had given me a bad taste in my mouth so far. hopefully the next one is MUCH better! I'll keep trying!

I thoroughly enjoyed getting to know Lord John Grey in his own right, rather than by way of Jamie and/or Claire. My interest in LJG was initially just as one of the better supporting roles in the Outlander series, but further interest sparked in An Echo in the Bone and in MOBY, leading me to follow his story back to the beginning - or as far back as Diana allows, at any rate. I'm so happy that I did.
adventurous mysterious medium-paced
Plot or Character Driven: Plot
Strong character development: Complicated
Loveable characters: Yes
Diverse cast of characters: No
Flaws of characters a main focus: No

Author: Diana Gabaldon
Genre: Historical, Mystery
Rating: A-

While I wasn’t particularly a fan of Diana Gabaldon’s first foray into the Lord John series, I was pleasantly surprised by its successor. In Lord John and the Private Matter everyone’s favourite gay, nineteenth century English lord is back to solve another mystery. This time, Grey is charged with investigating the death of one Tim O’Connell, a sergeant in their regiment and a suspected spy. O’Connell had been under watch since the regiments return to England but died before the papers he had stolen could be recovered – and the man watching him disappeared the same night that O’Connell died. While going about this, Grey is also investigating into the affairs of his young cousin’s fiancé, whom Grey believes has syphilis and wishes to find a way to end the engagement without creating a scandal that will shame his family.

I really liked the way in which Gabaldon tied the various plots of the story together. It’s not something I really expect in a Gabaldon novel; the Outlander books really aren’t known for their closure (although one hopes that by the end of the series things are all nicely wrapped up, with a bow on top), but Private Matter has plenty of closure. The immediate issues O’Connell’s death, espionage, and syphilis are all brought together and resolved in a manner that one probably wouldn’t have expected – in a manner that I really didn’t expect myself. Lord John isn’t my favourite of Gabaldon’s characters, I’ve always found him to be a bit stale and somewhat flat, and I usually can’t help but feel that his sexuality is just a new way for Gabaldon to introduce more sex, and with a variety of partners (I’m rather certain that of Gabaldon’s protagonists, John Grey has the largest number of sexual partners). I have no problem with gay characters, I just kind of roll my eyes when it seems apparent that the character was created so that he can have sex with this guy, and this guy, and this guy. In previous stories, Grey has always appeared to be one such creation, but in Private Matter he becomes more than just a slightly promiscuous gay English lord. His sexuality is actually a part of the plot, and for the first time Gabaldon divulges into the sexual underworld of London. I thoroughly enjoyed it.

The one thing that I did dislike about Private Matter, and really the one thing that I dislike about most of Gabaldon’s books, is the historical note. II love historical fiction and the thing that I’ve always enjoyed about books by authors like Bernard Cornwell is the way that, in the end, there is a historical note telling you about the real things relating to what you’ve just read: Cornwell’s can be a good twenty pages long, telling you what characters were made up and what are historical figures, as well as where he embellished, made changes, and made assumptions, and where he didn’t, making little notes of details of the plot. In Gabaldon’s novels (and her novellas) the historical notes are always lacking. For someone who puts so much detail into her writing, she is lacking in detail in her historical note and, while it doesn’t take away from the overall story, to me it’s a bit of a disappointment.

http://ishreviews.blogspot.ca/2012/06/lord-john-and-private-matter.html
adventurous mysterious fast-paced
Plot or Character Driven: Plot
Strong character development: Complicated
Loveable characters: Yes
Diverse cast of characters: No
Flaws of characters a main focus: No
adventurous dark funny informative mysterious medium-paced
Plot or Character Driven: A mix
Strong character development: No
Loveable characters: Yes
Diverse cast of characters: Yes
Flaws of characters a main focus: No