Fontspring is a website where you can buy fonts for your projects. No, just because you found a font online or on your computer for that matter, that does not mean you can use it in your book. You need to learn about font licensing and you will probably need to buy some fonts for various versions of your book. I personally use FontSpring, and only FontSpring when I want to buy fonts with the licensing needed to publish. They are fast at responding to questions and very clear about what each font they sell can be used for.