The Book module doesn't correctly use a fallback font for a character not supported by the current font. The workaround is to selected the text in the field, change to another font that supports the character, then change back to the original font.
1. In the Book module, click in a page-text field and set the font in the Type panel to Myriad Pro.
2. Copy this text: Boǧgazkale
3. Paste the text into the field and observe the ǧ is displayed with the last-resort box:
4. Select all the text in the field and change the font in the Type panel to Helvetica, and observe the ǧ is properly displayed:
5. Change the font back to Myriad Pro and observe that the ǧ is now correctly displayed using a fallback font: