I'm highlighting A Study in Scarlet Women, the first in her 'Charlotte Holmes' mystery series: it's fascinating and funny and very clever, with several interlocking mysteries. It captures the 'feel' and delight of reading Sherlock Holmes, but without the dated misogyny of the original that now subtly grates on me. I can't wait to read A Conspiracy in Belgravia, the second mystery in the series!
I also read her historical romances, My Beloved Enemy, Not Quite a Husband, and His At Night. Absolutely loved each one. Despite the 'breathless' covers, these are hands down the best romances I've read to date: smart, well-researched, and emotionally engaging. I loved the setting of the first two (colonial Chinese Turkistan and northern India, respectively), which I found fascinating and fresh, and the third (His At Night) is a clever 'spy vs. spy'-style romance that reminded me of everything I love about The Scarlet Pimpernel. Thomas is the latest addition to my favorite authors, and I'm in the middle of reading everything she's written.