Edward Conze's Buddhism: Its Essence and Development is among the most popular general introductions to Buddhism and for good reason. Conze is one of history's foremost European scholars of Buddhism. He translated numerous sutras and shastras and wrote works of his own on the history and philosophy of Buddhism.
The subtitle of Buddhism: Its Essence and Development aptly describes the character of the work. The early pages provide a brief listing, and account of the significance of the most important Buddhist documents and address some of the common questions and impressions about Buddhism that arose in Europe and the U.S., e.g., Is Buddhism atheistic and is it a pessimistic philosophy?
The core of the work describes the philosophical tenets of early Buddhism and those ideas that most all Buddhists accept, e.g., The Four Noble Truths. It also describes how different Buddhist schools of thought developed these ideas. In particular, the doctrines of the Theravadin, Sarvastivadin, and Mahasangikan schools are discussed and then later, he presents the doctrines of the Mahayana tradition (the Madhyamika and Yogacara schools). All of this might count as the "essence" of Buddhism.
Its "development" describes the full scope of historical Buddhism, including how it became transformed by the specific cultures that adopted it. Here the magical and mythological views that became attached to the core ideas are discussed. Conze also presents the views of Bhakti, Tantric, and Pure Land Buddhism. These seem to reside at the intersection between reasonable developments of Buddhist thought and the degeneration of its core ideas, brought on by the superstitions of the societies which produced these schools.
Perhaps the greatest (only?) weakness of the work is the brevity of the treatment that Conze gives to Ch'an and Zen Buddhism, making the work a bit too "Indo-centric" as a general treatment of Buddhism. At just over 200 pages, the work could have been much improved with an additional 30-50 pages on these schools of Buddhism.