I've been reading the Thornleigh series for quite a few years now. The first book got off to a rocky start with me, but then I really enjoyed it. The second book was great, but the third not so much. I am look forward to the 4th however, since it focuses on Isabella and Carlos - who made book 2 pop so much.
Below is a summary from Barbara Kyle's web page (http://www.barbarakyle.com/Books.page):
Young Queen Elizabeth I’s path to the throne has been a perilous one, and already she faces a dangerous crisis. French troops have landed in Scotland to quell a rebel Protestant army, and Elizabeth fears once they are entrenched on the border, they will invade England.
Isabel Thornleigh has returned to London from the New World with her Spanish husband, Carlos Valverde, and their young son. Ever the queen’s loyal servant, Isabel is recruited to smuggle money to the Scottish rebels. Yet Elizabeth’s trust only goes so far—Isabel’s son will be the queen’s pampered hostage until she completes her mission. Matters grow worse when Isabel’s husband is engaged as military advisor to the French, putting the couple on opposite sides in a deadly cold war.
Release date is listed as August 30 on Kyle's page, but Chapters has it as being the 23rd.
Looking forward to reading it!
Update of September 1, 2011:
So, I've read it, and it was great! Like I predicted, I enjoyed it because of the Carlos/Isabel story line. I won't be including a full review of the work, since it is number 4 in a series, but I HIGHLY recommend that you pick it up.