An ABC Summary or ABC Diary focuses on finding something for each letter of the alphabet that can tell you something about the book. U, X, Y, and Z are pretty difficult. One way to begin is brainstorming lists of words that begin with these letters. Of the four, I think U and Y are the easiest. You can use “until” and “yet” with plot points for these letters.
That leaves X and Z. It is difficult to think of words for these letters. A good one for Z is “zeal,” which means really wanting something. For X, consider “x-ray.” For example:
X-rays of Old Dan would reveal that he was injured badly and dying.
After the death of the two beloved dogs, the story has a happy ending.
In spring, when they were found, a beautiful red fern had grown up between their two bodies. (p. 210)
You have enough information now to tackle those last letters!