I found Apple Books for Mac particularly useful with handling free samples. I buy ebooks in a couple of ebookstores. They offer free samples, so I pick up epub format and save it to the desktop to read a few pages before deciding whether I want to purchase the title.
Some sites (such as Project Gutenberg) have clear URL addresses of book files. When you tap on a link to the epub format (ending with .epub), you should be able to open it with Apple Books app on your iPad or iPhone. 2b1af7f3a8