Added a new category for Books

[stjepanjurekovic]

Hi,

Stjepan from Manning here. We've got this awesome new book on CSS written by Michael Gearon and Martine Dowden as a part of Manning's early access program. I thought it might be a good match for your list. Therefore I added a new category "Books". Thank you for considering it.

Best,

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By submitting this pull request, I promise that:

• [x] I have read the contribution guidelines thoroughly.
• [x] I ensure my submission follows each and every point.

[mischah]

But I‘m happy to hear other opinions 👍🏻 / 👎🏻