If you bought the ebook on a Mac, the easiest way to do this is:
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1. Download the EPUB file.

2. Double-click it.

3. It will open in Apple Books.
4. Create a Collection in Apple Books called Leanpub by clicking + New Collection in the My Collections section on the left

5. Drag and drop your ebook into the Leanpub collection.

6. On your iPhone or iPad, the Leanpub collection will be there. Tap it to open it.

7. Tap the book, and the ebook will download.

8. Tap the book again to open it.