Histidine aromaticity
This is one of my favorite charts demonstrating the complexity of amino acid properties:


http://www.jalview.org/help/html/misc/aaproperties.html

Histidine is probably the most complicated amino acid in this regard (just compare how many circles it falls into).  But don't undersell Cysteine and Methionine; those sulfurs exhibit some surprising behavior, especially when coordinated properly.  Cysteine of course forming disulfide bonds, Methionine occasionally functioning as a methyl-transferase. 

Biochemistry text books make some over simplifications because presenting the full messiness of these classifications is pretty confusing. Lehninger itself is the worst about this (and especially when presenting math).  Voet and Voet is probably the most reliable about presenting this complexity. I still think Lehninger is a better teaching textbook overall (I was first taught Biochemistry from the 3rd edition), but I also recall spending a week being confused about a topic before someone pointed me at V&amp;V and I realized Lehninger was oversimplified to the point of being wrong, in that case.