Hi Frankie, I have neither outlook mail or a kindle so take this answer as you want.. however "normally" after receiving an email (in outlook) you would open it and download the attachment accompanying it - presumably in this case your book.
Then you go to the folder/file you downloaded to and open it to read as kindle.
If it is not an attachment but a link then clicking on the link (IF you trust the sender of course) should provide the same option of saving to your device.
The "sort of" normal way is, and this is not specifically for Outlook file attachments :
Open the Kindle app on your device.
Go to your Library.
If you're on a computer, double-click on the book cover. If you're on a mobile device, select the book cover.
Maybe a mixture of these will help you, good luck 🥰