There are a few books that I think should be in every artist's library. In addition to Julia Cameron's phenomenal The Artist's Way Series, I would also recommend The Artist In the Office become a fixture in the collection of every creative person working a day job to pay the bills.
The talented Summer Pierre, who is a writer, artist, and musician, wrote and illustrated this charming book. In it, you'll find inspiring quotations, like this one:
So much of our creative output depends upon our own outlooks and attitudes, and I think this book inspires a level of optimism and positivity that every creative person needs to continue to do great work, both on the clock and in our artistic pursuits.
You can find The Artist In the Office at numerous online retailers. It makes a great gift for the artists in your life.