Textbooks

1. "Verilog HDL: A Guide to Digital Design and Synthesis" by Samir Palnitkar, Prentice Hall.
2. "Digital Design" by Morris Mano, Pearson Education.
3. "Principles of CMOS VLSI Design: A Systems Perspective" by Neil H.E. Weste, David Harris, Ayan Banerjee, Pearson Education.
4. "CMOS: Circuit Design, Layout, and Simulation" by R. Jacob Baker, Wiley-IEEE.
5. "The VLSI Handbook" by CRC Press.

Verilog Tutorials & Guides

6. Verilog Tutorial by Deepak Kumar Tala
7. Verilog tutorial based on Weste and Harris, edited by Lukasz Strozek

Simulation Tools

8. Ngspice - Open Source SPICE Simulator
9. Qucs - Quite Universal Circuit Simulator
10. KiCad EDA - Integrated Open Source Suite

Additional Web Resources

11. All About Circuits - ALU Resources
12. CMOS Inverter Theory - Electronics Tutorials