In general, I would recommend learning about typography. The elements of typographic style by Robert Bringhurst is one that I liked as a typographic reference. I'm reading typographie by Emil Ruder and Display Typography by Erik K Bain right now and I would recommend the latter.

Design Thinking by Ellen Lupton will help with concept development, sketching, and research. I'm reading Graphic Design Process by Skolos & Wedell right now and I would recommend that too.

If you ever want to learn more about advertising, creative ideas, strategies and concept development, read The Advertising Concept Book by Pete Barry, it's surprisingly big.