In the past week or so I have completed books 1, 2, and 3 of the Level Up series.
I downloaded all of the answer PDF files available for the books and installed them on my iPad under the iBooks app. This makes it easy to check my answers. I go through a book one time without looking at answers and then I go through it one more time checking one page at a time against the PDF file.
I am currently working my way through book 4 of the series.
I notice that books above level 10 are currently available, but they do not appear to have PDF answer files ready for those volumes yet. I hope they will by the time I work my way through volume 10, which is as high as I have purchased so far. I won't buy any higher until the PDF files are available.
While answers to books 1 through 5 appear to be really easy, I imagine that above 10 the answers will be harder.
I am really enjoying these books.