If you're 60% through your novel, then that means you've already put in a lot of hard work, so why not continue with till the end? Personally I try to not think about other book ideas until I'm at least nearly finished with my WIP. Also, when you say 60% through, do you mean 60% through with your first draft? If so keep in mind there's yet a hard road of work ahead of you in the editing/revising process, so you may not be 60% done, but instead more like 20% done (or even less). Regardless, focus on your current book and try to push all other ideas aside. It's not that hard, at least not for me. Working on a book requires much concentration and brainpower; use your energy wisely. But if you have some really cool ideas that can somehow fit into your current story, you can try that, too. Good luck!