If you like reading Jack Reacher books,keep reading them but pick 1-2 books from the list above also for style variety and to prepare yourself for the style of reading you’ll have both on tests and in college (especially nonfiction titles).
You can prepare for the ACT starting now. You can review SAT math on Khan Academy’s website (will be useful before 1S anyway). You can buy “Up your score” and have fun with it in whichever way you want. But the most important things you can do this summer are 1° read in English 2° gather all possible information about the TPE science project and what your school offers 3° read this website, get as much information as possible from various sources (The Princeton Review’s Best Colleges would be an easy introduction to roughly the top 10% colleges in the US, the books AdMission Possible! or The College Solution are good books to understand the process, you can create a tryout commonapp account on August 1st to see what it looks like - that account will disappear in July of the following year anyway so you’ll have a “real”, “clean” one for your “real” application season).
If you feel that you missed out on subjects, you can learn about them on MIT’s OCW courseware for high school students as well as Khan Academy (lots of good information there).
The TPE doesn’t need to be “a success” - you’ll be given a limited projet to carry out and you’ll need to document, create, solve problems, present findings, etc. Obviously no one expects 11th graders to make a major scientific discovery :D… the process is what matters. You can treat it as a BS project, go through the motions… or spend all your free time in the day trying to work out the problems with your experiments.
Read the book Science Fair Season!!