AHDL Syntax Manual

I found just searching for "AHDL" in the quartus help can be quite helpful.

I have found having a good knowledge of another HDL helps, having done reading/translating of AHDL. Its quite straightforward and only really a netlist. You do have to get used to the idiosyncracies, like any .d or .en input on a register is set to GND by deafult when not set in a given state, when in VHDL it would hold value. AHDL wants you to be quite explicit.