I love Amelia, Radcliffe and the Master Criminal.
I think the Master Criminal is the best adversary you could ask for in a murder mystery/whodunit. His relationship with Amelia is fantastic, and the jealousy Radcliffe exhibits towards that relationship is even better.
I hate including spoilers because I see my review as helping others to decide if they want to read the book or not. My suggestion you MUST read at least one Amelia Peabody book in your lifetime. Start with Crocodile on the Sandbank and if you don't fall in love with Amelia and Radcliffe by the end of that book, you will not get the rest of the series.