Hi there. My name is*****'m sorry that you are having this trouble and I will do my best to help you. Let's check to see if you may be running low on storage space on your phone.
Please let me know the results. If you don't have much space left, please try deleting some of the books that you have already read, and/or any apps that you aren't using. Photos and music take up a lot of space as well. If the problem is the result of too little free space, deleting some things and freeing up space should allow you to then download the other 2 books.
Please keep me posted and let me know how its going. Thank you!