Hi Anee. I believe setting the scene helps you understand these sentences.
Suppose you are going to buy a book from the bookstore, and I ask you if you got the book. If the book is a rare one or limited edition and it's hard to get one, then you would answer like this: 我还没买到书呢。By saying so, you are indicating that you may be standing in line, waiting, and there's a chance that you can gat the book eventually. In other words, buying the book IS what you are going to do, it's certainly on your to-do list.
However, if you were lying on the sofa, and your parents are shouting that you should do something immediately. But, you can't do your next step without a book. In that case, you would say, 我还没买书呢。This sentence suggest there's something behind "buying the book".
I think the difference between the sentences lies in is there something to do after buying the book.
Hope my answer inspired you. If you have any questions, leave a comment below and I will answer that when I see it. Have fun learning Chinese.