Great historical conspiricy books

I'm a Tudorian buff. Movies and tv series like: A Man For all seasons, movie with Paul Schofield and TNT play version with Charleston Hesston. Numerous Mary Queen of Scots movies, I love the one with Glenda Jackson. Anne of the Thousand Days with Richard Burton and Genevieve Bujold.  Elizabeth R, the BBC series shown on PBS with Glenda Jackson. HBO's Henry VIII series.  The list is quite long.
I've read numerous books in the time period. The Last Plantagenets by Thomas B. Costain (last of the 4 volume set). Naked Before Mine Enemies by Charles W. Ferguson, The Autobiography of Henry VIII by Margaret George. My Enemy the Queen by Victoria Holt. Elizabeth the Great by Elizabeth Jenkins. And many others.
I did a major paper on Cardinal Wolsey in college. In my trilogy of Fletcher Family Battles, the third one is the Battle of Flodden which has Wolsey as the Kings Almoner before he become Chancellor.
I recently came across two books set in the forgotten part of Henry VIII's reign. It deals with his 5th wife Catherine Howard. The second one history records as beheaded.

The Catherine Howard Conspiracy by Alexandra Walsh.
The book starts off in the present day with Perdita Rivers archeologist working one a sunken ship dating from Tudorian times. She's pulled away from the site because her long estranged grandmother dies. She and Piper, her twin sister, are named as sole heirs of Pembroke shire, her estate in Wales.
Her grandmother, Mary Fitzroy, had been a well known author on the Tudors and other parts of English history. Going through her papers and unpublished manuscripts she finds Mary came across of number of anomalies concerning Catherine Howard. Not wanting to spoil things too much, but history records that along with Catherine, Thomas Culpepper was her lover and was executed as well. His tombstone has him dying six months before the execution. Food for thought.
 She came to the conclusion that the fifth wife of Henry VIII did not die, but lived in hiding at the ancestral home of her dead cousin Anne Boleyn, namely Pembroke shire.
Thus begins the great what if? that fuels the story. What if Catherine didn't die, but escaped and history records as being executed to hide the fact. What if when she fled her mentally ill and abusive husband she was pregnant. What if she delivered twins, a boy and a girl. If this became known how would it have changed history? How would it change the present if this became knowledge?
The story has two time periods, the present where Perdita and Piper are going through historical records to prove their hypothesis while a mysterious government agency is trying to stop them at all costs.
The past that tells the story of a frightened fifteen-year-old girl thrust into a marriage by her power hungry uncle that's head of the Howard family and want to join the Howard with the Tudors.
Each time periods is an interesting story in itself, what I enjoyed is how the present day scholars conclusions as the read the documents and examine the artifacts are sometimes right and other times way off what actually happened.

    There is also the second book, with a promised third on the way.
If you accept the premise that Catherine Howard wasn't executed and was the mother of a girl and a boy. What happened to them?
In this book the head of the mysterious government agency that was trying to stop their research is replaced as he overstepped his authority. A cousin of Perdita and Piper has twin daughters. The inheritance of Pembroke shire can only be passed down to daughters. This cousin tries to kill Perdita and Piper so his daughters can inherit.
Meanwhile they dig even deeper into Pembroke archives searching for any clue that in the time period Catherine lived there and there were two infants. This leads them to another startling conclusion as to who really was Mary Queen of Scots.
Oh the intrigue in both past and present. Oh the possibilities, oh the probabilities. I can't wait for the next book.

An aside on how good these books are: I recommended these books to my mother. She lives in a senior living facility. She wanted me to pick her up and go shopping. When I arrived she said she couldn't go as she didn't get any sleep that night. And it was my fault because she couldn't stop reading the first book.