24V voltage regulator from ~55V/5A
If you use a linear regulator, you're going to waste more than half of the available power.  I would recommend some sort of switching converter.  That's a pretty high input voltage, though.  I don't know of any good switchers offhand that will run with a 60V+ input voltage.  

I would suggest taking a look at the power converter availble from Linear and TI.  It looks like linear has a few that will work:

http://parametric.linear.com/External_Power_Switch_Buck_Controllers#!1646_Buck!1032_%3C=50!1033_%3E=60!1105_%3C=24!1034_%3E=24!1107_2:15!vinmin_50!vinmax_60!vout_24!iout_2

The LTC3810 looks like a good candidate - it will work on input voltages up to 100 volts and it can provide output voltages from 6 volts to 55 volts.  

http://www.linear.com/product/LTC3810

It does take a handful of external components to work, but the datasheet walks through the calculation of the various passive components.