Why are highly branched alcohols solid? Usually branching (or decrease in surface area) leads to increased volatility. But in case of alcohols, my book states that:


  The higher alcohols (butanols to decanols) are somewhat viscous, and some of the highly branched isomers are solid at room temperature.


Here the decerease in surface area did not increase the volatility; on the contrary, volatility has considerably reduced. 

What is the reason of this anomaly? Or is this even an anomaly?
Well, if you're talking about melting points, the simple argument that more compact molecules should have higher mp because of increased dispersion forces can easily fail.  The problem is that melting is a much more complicated phase transition than boiling, with significant entropic effects coming from considerations of packing and entanglement -- especially relevant for floppy large molecules -- so that arguments based purely on the enthalpic changes due to intermolecular forces can easily be insufficient.