Ask HN: How to Get Started with Embedded Software Development?

I am a student that has completed one year of CS.

Each summer I work at a small manufacturing company. To date, I have completed a number of projects with them. These include Excel VBA, static website development and some prototyping using Arduino.

I recently got asked to work on a PIC project with them. It would invole tweaking and modifying existing code (no hardware changes). However, I had to decline as the project was above my knowledge and ability. I knew I wouldn't get the project completed by the desired deadline.

Needless to say, the boss wasn't too happy, as the project will now be more expensive as it will be outsourced.

How can I make the jump from Arduino to programming the likes of PICs? Recommended books, guides, process?