It completely depends upon where you are in your journey / quest for some semblance of sanity to be honest OP. I tend towards books find us, rather than the other way, so suggestions may not work as well as you putting the leg work in and perusing the relevant section in your local book store, if one still exists where you are.
Having said that, there are some good suggestions above, particularly The Power of Now and The Road Less Travelled, although both will explore one’s relationship with faith, which quite frankly is one of the cornerstones of inner peace IMHO.
If you, like nearly everyone on here, has a problem understanding the difference between religious dogmas and personal faith, then the two books by Eckhart Tolle and M Scott Peck will test your powers of endurance. Alternatively, if you approach your journey and life with an open mind, you’ll no doubt find them beneficial.
Another small tome I found, and still do, very useful is Heal Your Body by Louise Hay, this is a quick reference guide to the metaphysical causes of symptoms, which does actually make you think…
Good luck with your journey and bear in mind that age and experience are two powerful factors if you’re so inclined.