To download books from Amazon you need the Kindle app, which is free. I don't know the Hudl but on my iPad I read books on kindle, kobo and iBooks. I log into Kobo as my daughter and that way we can share books, and the cost of them.
I've bought all of my books for my Kindle from Amazon, about 70 so far, most were free, some were 99p or thereabouts. I would recommend that you read the reviews of any book from an author that you are unfamiliar with, and I buy nothing with a star rating of less than 4/5.
Also, you can - at least with the Kindle - sync to your iphone which is really handy to catch up on a few pages here and there
I also use Bookbub which is variable in the quality of books offered. If I don't know the author sometimes they are self published and can be pretty dreadful . If the publisher prices a book at £8.99 and then a special offer for 99p it may well be a good buy,sometimes it's the second in a series to get you hooked if you see what I mean.