The following are a set of links to websites and resources that I think are valuable or interesting. The topics and links are listed in alphabetical order.
Communication Systems
Introduction to the Controller Area Network (CAN) - Steve Corrigan
Optimisation
Engineering Design Optimisation - Joaquim R. R. A. Martins and Andrew Ning
Radio
Fraction/Integer-N PLL Basics - Curtis Barrett
High Frequency VCO Design and Schematics
PLL Performance, Simulation, and Design - Dean Banerjee
Software Abstraction
Architectural Styles and the Design of Network-based Software Architectures - Roy Fielding
Designing abstractions with properties in mind - Ted Kaminski
The one ring problem: abstraction and our quest for power