It is the best and greatest VHDL book I ever found. CAST Club . They provide free VHDL models monthly. You can download and study these models. You must register before you can download them.
Registration is free.
VHDL Examples for Synthesis by Chang . No demo is available but it is affordable for individuals. VHDL Technology Group VHDL FAQ FPGA Resources FPGA Workout II . Better check it out. Multi-FPGA Systems , Ph.D. dissertation by Scott A. Hauck.