What does the warning sign ⚠ on my book in the library mean?

In FBReader 3.8 and newer, this warning sign indicates that the book appears to be on your device, but the app does not have access to the file. You can resolve this issue in one of the following ways:

• Manually select the file: Tap the book and choose the corresponding file. When you select a file from the file system, you grant FBReader permission to read it.
• Add the folder to the library path: Go to Settings → Library and add the folder where the book is stored. This grants FBReader access to all files in that folder, including future additions. However, note that not all folders can be added due to Android's security restrictions.
• Move the book file to an accessible folder: If the folder cannot be added to the library path, you can move the book to another folder. For example, the Download folder is often restricted, but you can create a subfolder like Download/Books and move the file there. After moving files, run Rescan from the library menu to update the book list. FBReader will automatically link your moved files to the correct books.