The Urantia Book consist of four parts. As the often-seen cover says, the book reveals the mysteries of God, the Universe, world history, Jesus, and ourselves. Below, Amazon says:
You have just discovered the literary masterpiece that answers your questions about God, life in the inhabited universe, the history and future of this world, and the life of Jesus. The Urantia Book harmonizes history, science, and religion into a philosophy of living that brings new meaning and hope into your life. If you are searching for answers, read The Urantia Book!
The world needs new spiritual truth that provides modern men and women with an intellectual pathway into a personal relationship with God. Building on the world's religious heritage, The Urantia Book describes an endless destiny for humankind, teaching that living faith is the key to personal spiritual progress and eternal survival. These teachings provide new truths powerful enough to uplift and advance human thinking and believing for the next 1000 years.
A third of The Urantia Book is the inspiring story of Jesus’ entire life and a revelation of his original teachings. This panoramic narrative includes his birth, childhood, teenage years, adult travels and adventures, public ministry, crucifixion, and 19 resurrection appearances. This inspiring story recasts Jesus from the leading figure of Christianity into the guide for seekers of all faiths and all walks of life.

How to read the Urantia Book - a personal suggestion
1st: Due to the complexity or in-depth description it is recommended that you do not start at the beginning, but rather re-visit it after gaining some understanding from some of the other papers This will help you from becoming overwhelmed or discourage due to the deeper subject matter, but by all means read it and get involved with it as your understanding widens and grows.
Most readers I have heard of and the advise I give is to start with Part IV, The Life and Teachings of Jesus. Thus…
2nd: Recall a story of Jesus from the Bible, perhaps even one from Sunday School, and find it in Part IV of the UB. For example, the turning water into wine miracle, or the feeding of the 5000. Or the crucifixion and resurrection of Jesus. Or the “Last Supper” or the “Sermon on the Mount.” Read one or more of those to get a feel for the language and the detail of the content.
3rd: Think about what you read and how it was the same or different from the Bible. Why was it different, if it was? How did it make you feel.
4th: Keep and open mind and look for the Truth in what you are reading and understanding. There is so much more in the Universe than we have been told and led to believe. The Urantia Book will help you understand this and much more.
5th: After you have read some selected papers or sections in Part IV, you may wish to read the entirety of it. While you are reading, you will come across references to other ideas that you can explore in the other parts of the book. From here, you’ll know what to do. Some go right to Part I and jump at this point, usually without feeling overwhelmed. Take your time. Or you may want to start with Part III, The History of Urantia, before you go to part I.
You may want to find a friend who can read it with you and talk about it. I have given away many of the book to friends; we talk about it and discuss ideas from it. There may also be a study group in your area; those are always helpful in gaining more insight into the book. There are vast resources on-line about the book, including the entire text (which can be download or read on-line), audio files so you can listen to it being read or download the MP3 files (for example, urantia.org and truthbook.com. Amazon has many other books connected to the UB. I can supply a list for those interested.
Turning water into wine (UB Paper 137, section 4); the feeding of the 5000 (UB Paper ; the crucifixion (UB Paper 187) and the resurrection of Jesus (UB Paper 189); the “Last Supper” (UB Paper 179); the “Sermon on the Mount” (UB Paper 140).
For more info: UrantiaBook@unseen.is
Who are the authors and how did the Urantia Book come into physical being?

The best book on the overall history is A History of the Urantia Papers by Larry Mullins and Dr. Meredith Sprunger. In this book you will learn all there is to learn about the strange authorship of the Papers and how they came into being, including documentation that support the material. It is worth the reading.
But here is a chart of Dr. Sadlers reasoning on how the book acturally became a physical, material work, as received from the celestials.
Are we children of God, or simply complex animals? Will we survive death, or are we doomed to extinction? Is humankind an accident, or are we part of a grand universe plan? The Urantia Papers give persuasive answers to these questions. Over 500,000 copies of The Urantia Book are now in print. But, who wrote it, and by what authority? More intriguing and thrilling than any science fiction or mystery story are the actual events that were set into motion in Chicago nearly 100 years ago and eventually resulted in one of the most astounding publications ever printed. However, until now, a carefully documented history of these Papers, where they came from, and how they became published as The Urantia Book, has not been available. In A History of the Urantia Papers, Larry Mullins and Dr. Meredith Sprunger have collaborated to strip away prejudice and superstition and unfold the facts about the origin and history of the Urantia Papers. Whether you are familiar with the Urantia Papers or simply curious about them, you will find A History of the Urantia Papers a captivating and challenging story. — Amazon description