Buddhism, unlike other religions, encourages mysticism. If you’re a Christian and you try to be a mystic, you have to be careful otherwise people will call you an occultist or something. Islamic mysticism, Sufism, often gets ostracized and politicized. But Buddhists built it right into the religion.
All mystic traditions have a conceptualization of enlightenment. This is because all mysticism is basically the same, your progress through a series of meditative steps until you earn an experience of ultimate reality. This is what separates mysticism from ordinary religion, rather than argue over what’s true, mystics just go out and experience it for themselves.
Enlightenment is for everyone. It really is. Mystics spend vast amounts of time trying to understand and then communicate knowledge about the things they’ve figured out. Buddhists call this dharma. Hundreds of people have reached enlightenment and then write dharma books of their own. Dozens of them are even known in the West.
My recommendation for a dharma book is Mastering the Core Teachings of the Buddha. It’s written in easy-to-follow, untranslated English, and uses Western concepts to communicate the understandings. The author is serious, knowledgeable, and quite capable. It’s the only one you really need, but of course if you want to get serious about Buddhism, you’re going to eventually want to go to the source.
It’s important to note that any book on spirituality is essentially a dharma book, and all the teachings and techniques are just variations on the same core teachings, concepts and experiences. The Buddhist conceptualization of spirituality and mysticism is again, just one. The Buddha didn’t invent anything. If you start picking up books written in different traditions, you’ll start to see the same concepts over and over again couched in different terms. Rest assured that there’s nothing “special” about Buddhism other than that it has less baggage and internal cruft than other traditions.
That said, what is enlightenment? Well, as you progress in mysticism, you earn consciousness states. The states can be transitory, or they can be longer-lasting. Both have value in Buddhism, but enlightenment is traditionally defined as a progression through a number of the longer-lasting states, which you achieve through meditative practice. The experience of passing through the states provokes an irrevocable change in your being. That change is enlightenment.