I remember the first time I tried to read an attached book on my Kindle, I was a bit confused but figured it out quickly. If you receive a book file via email or download it from a website, you can transfer it to your Kindle by connecting your device to your computer via USB. Once connected, open the Kindle folder and drag the book file into the 'documents' folder. If you prefer wireless transfer, you can email the file to your Kindle's unique email address, which you can find in your Amazon account settings under 'Manage Your Content and Devices.' Just make sure the sender email is approved in your settings. The book should appear in your library shortly after sending. For formats like EPUB, you might need to convert them using a tool like Calibre or Amazon's Send to Kindle service, which handles the conversion automatically.