General

GameStationX's Hardware Book — lots of good info

OpenCore's suite of hobbyist design tools

Techniques

Home PCB Making — Howard W. Sam's guide

Soldering — one, two

Debugging — one

Prototyping with Surface Mount — one

Wirewrapping — one

Part polarity/pinout/symbols

Integrated Circuits — one

Diodes one

Electrolytic Capacitors — one

Transistors — one, two

Packages — one, two, three

Pinouts

pinouts.ru, allpinouts.org, hardwarebook.info

Specific things

• All about driving LEDs, and multiplexing 7-segment displays via just a PIC
• How to drive 4 7-segment displays, and use 3 switches with only 11 pins
• How to drive 56 LEDs with 8 pins
• What the various components in a voltage regulator are needed for