iBooks Author can embed custom fonts into books for complete control over text appearance. Only TrueType (.ttf) and OpenType (.otf) fonts are supported for embedding. The Font menu in the Format menu bar will only show supported fonts, but the OS Font panel (accessed by clicking the Fonts button in the toolbar) should display all of the fonts on your system. You will receive a warning if you select an unsupported font, and a supported font will replace it when you view the book on an iPad. For best results, use the Font menu in the Format menu bar to select the fonts in your book.