This is an excellent introductions to the key ideas in DSP. There are 4 chapters, each with several parts, but all are concise and packed with information. Some of the more tricky concepts have great animations to help bolster the explanations.

Thoroughly recommended. link

1. Trigonometry - the language of oscillations,
2. Complex Numbers - the power of 2 dimensions,
3. Signals - from math(s) to signal processing and
4. Systems and Convolution - what happens when signals pass through systems.