Reddit mentions of Designing Better Maps: A Guide for GIS Users

Find and read everything you can by Dr Cynthia Brewer. Start with Designing Better Maps: A Guide for GIS Users. I've had to re-buy most of her stuff two or three times as they mysteriously disappear from wherever I'm working.

Have a look at typebrewer, it's tools are broken right now, but the sites a good read.

Have a read of some it the stuff from the Google Maps blogs, and the blogs about Google Maps.

There's a couple of good YouTube channels out there talking about graphic design for web site or UI coders. While they're not about maps, most of the concepts are very relevant.

To be completely I honest, it is adequate work for a GIS class and I have seen worse cartography from "trained" GIS professionals. I use to work for an assessor office in a US city with nearly 250k people and the IT/GIS and planning depts gave maps of poor quality to a well known non-profit for a presentation. I am no cartography expert but it is one of the easiest things to criticize/fix for most maps. If I count my grad school job, I have been working with GIS for 6 years and I seen/made plenty of bad maps from cartographic point of view. Probably most in this subreddit. I think good cartography can make a newbie GIS professionals stand out if they are looking for work samples.

If you have the money:

Cartographer's Toolkit: Colors, Typography, Patterns

Designing Better Maps: A Guide for GIS Users

Cartographic folks might use these resources: http://colorbrewer2.org/ , http://typebrewer.org/ and https://www.amazon.com/Designing-Better-Maps-Guide-Users/dp/1589484401/

Especially PSU students.