The following books have made an impression on me and my beliefs.
Biblical Literacy by Rabbi Joseph Telushkin. This book gives you a synopsis of the bible, and Telushkin draws your attention to the moments and ideas that are important in Judaism. In a similar vein, How to Read the Jewish Bible by Marc Brettler, also surveys the content of the Bible and helps to put it into a modern Jewish perspective. Robert Alter’s translation of the Hebrew bible The Five Books of Moses, as well as its continuation in several more volumes, is also a vital resource to me. Alter brings an incredible amount of knowledge of Hebrew and related languages to bear on his translation and has copious notes about the decisions to be made, as well as point out puns or inside jokes that would not be apparent in the English translation. Bringing this edition to Bible study is worth a lot. Etz Hayim is the Conservative movement’s official chumash; the translation is bland, but you get 2:1 notes to text, which is wonderful, including a lot of very interesting tangential material. I found the combination of these two books to be very effective for bible study.
In a different vein, Chuang Tzu, translated by Burton Watson. The inner chapters in particular are both powerful and beautiful, but there are lots of gems in the remaining chapters as well, if you can ignore some repetition. I find a lot of compelling ideas in Taoism, especially Chuang Tzu’s version of it, but there is beauty in Tao Te Ching, the Addiss/Lombardo translation being the one I have.
The Rise of Christianity by Rodney Stark. This book shines a light on the darkness between 70 CE and the moment Christianity became the religion of the Roman Empire. Where did it come from, how did it grow so quickly? This book gave me a deeper appreciation of what it is about Christianity that made it change the world, and gave me more compassion for my Christian friends. Similarly but for Islam, Introduction to Islam for Jews by Reuven Firestone clarified things greatly, by showing ways that Judaism and Islam are similar and different. I think this book would work with a non-Jewish audience.
The Other God by Yuri Stoyanov is a fascinating deep-dive into the history of dualist thought, from ancient Egypt, through Zoroastrianism to defunct and mostly lost religions like Manichaeism and Catharism. Similarly, Ancient Mystery Cults is a deep dive into Mithraism and the Eleusian mysteries, what is known anyway, and the book was an effective antidote to tropes like “Christianity is just Mithraism dressed-up.” The Indo-European ideas that reappeared in Mithraism were also the bedrock of Iranian religion, where the dualist religion of Zoroastrianism was born and many other religions underwent change: the topic of the book Religions of Iran by Richard Foltz. Although not so much about religion, Genghis Khan and the Making of the Modern World by Jack Weatherford illustrates how many modern ideas were born by the Mongols and the fruit of their conquest, in the neighborhood.
In the Dust of This Planet and volume 2 of the same series, Starry Speculative Corpse are excellent overviews of a lot of areas of inquiry. I hesitate to come down hard as a pessimist but negative theology conceptually is very appealing to me and my understanding of it comes from these two books, which are short and approachable. I also got a lot out of Emerson, especially “Self-Reliance” and “Circles.”
Two other philosophers made deep impressions on me: Henri Bergson, whose Creative Evolution kept me struggling in college but whose ideas about time, duration, evolution and intuition have stuck with me in some bastard form, and Jiddu Krishnamurti, who really only had one thing to say (paraphrased “you don’t need a teacher, which is handy because you can’t have one anyway”) but said it so audaciously and powerfully that he wound up attracting a following and having to say it repeatedly. Try Freedom from the Known. (He deserves bonus points for dissolving the cult he was brought up to lead.)
In more depth with Judaism, we must discuss Heschel, whose books are so powerful I haven’t finished any of them. God in Search of Man is the one I most want to finish, The Sabbath is one I actually have finished, I Asked for Wonder is to my recollection, quotes, but even his quotes are great.
Some dimensions of Jewish practice that are worth discussion: Everyday Holiness teaches the practice of mussar, which is a form of Jewish-religious self-help; an interesting topic for someday when I’m bored. Jewish Meditation is what it says on the tin, and Rabbi Kaplan also did translations of other important Kabbalistic works, especially the purported manual for making golems Sefer Yetzirah and proto-Zohar Bahir, both of which are only approachable thanks to his copious notes. Notes outweigh source matter by about 10:1 in each of those.
Some more comparative religion, The Sikhs by Patwant Singh is a nice overview introduction to the religion I read before Wikipedia. Ofudesaki is the holy text of a fairly obscure Japanese religion, Tenrikyo; I never made it further than about 10 pages.
More crap—I mean, stuff—in the Western esoteric traditions: The Secret Doctrine of the Rosicrucians explains their “philosophy” of vibrations and magnetism and whatever. Similarly, Alchemy Rediscovered and Restored gives you more of the wider story of “spiritual alchemy” rather than just the Rosicrucian “version” of it. Feel free to go full stupid with Practical Qabalah and The Mystical Qabalah, but be sure to pick up Aleister Crowley’s books at some point.