Hands down the best of this series so far. Winter is an an excellent hero; strong, silent, dry-witted and steadfast. Plus he is dead sexy, AND a virgin (are you stunned? I was!). Isabel is an amazing heroine. Smart, generous, sweet and very no nonsense but in a good way. I think that I have an affinity for a more worldly women in hist/roms, usually widowed or labeled as spinsters, because they often don't have the same strict society strictures placed on them. They are allowed to be strong willed, bold and more forthright.
The mystery aside what I really enjoyed was the slow reveal of the Ghost of St. Giles' identity. While you know that Isabel will figure out what we all already know the progression and how entertaining it is when she gets there truly made Thief of Shadows my favorite of the Maiden Lane books so far.
ps. kudos to Hoyt on the clever set up for book 5, I can't wait to read it.