The "marvelous work and a wonder" doctrine comes from Isaiah, Therefore, behold, I will proceed to do a marvellous work among this people, even a marvellous work and a wonder: for the wisdom of their wise men shall perish, and the understanding of their prudent men shall be hid….And in that day shall the deaf hear the words of the book, and the eyes of the blind shall see out of obscurity, and out of darkness. The meek also shall increase their joy in the LORD, and the poor among men shall rejoice in the Holy One of Israel (Isa 29:14,18-19). What is marvelous and a wonder is the work the Lord performs in the last days. Verse 18 above shows that the Book of Mormon plays a role in the process but is not the "marvelous work and a wonder." The phrase means that the Lord will build a kingdom in the latter days using the simple and meek of the earth. He will gather the tribes of Israel by a great missionary effort that will be successful because of the Book of Mormon and other latter-day scriptures. LeGrand Richards explains how crucial the ministry of Joseph Smith was to this process.
LeGrand Richards
"Joseph Smith, or any other man, could not have obtained all this information by reading the Bible or studying all the books that have ever been written. It came from God. It is exactly what Isaiah promised the Lord would do when conditions upon the earth should become as Joseph Smith found them when he went into the woods to pray for light as to which of all the churches he should join….
"It truly is a marvelous work and a wonder. Can you conceive of anything that could be more marvelous or more wonderful?
"In the face of it all, the wisdom of wise men does perish. The world has no satisfactory explanation. In the Church Historian's Office in Salt Lake City are more than 20,000 volumes, large and small, each of which says something about the Prophet Joseph Smith. There are also some 2,000 pamphlets on the subject. Many of these publications represent attempts on the part of non-Mormon writers to explain the conundrum of Joseph Smith and the work he established, but all without avail. All these writings have been accumulated since the birth of Joseph Smith in 1805. In contrast, in the more than two centuries that have elapsed since the birth of George Washington, it is reported, there are only some 3,000 volumes written about him on file in the Library of Congress….
"We emphasized a statement published in the New York Herald, which we quoted in this chapter; 'Joseph Smith is creating a spiritual system, combined with morals and industry that may change the destiny of the race.' This statement was made in 1842, and at this writing, more than a century and a half later, it is evident, from the present accomplishments of the Church founded under his leadership, that this prognostication is sure to achieve a complete fulfillment.
"It is equally sure that the prediction of Josiah Quincy, previously quoted, will find its fulfillment:
"'It is by no mans improbable that some future textbook, for the use of generations yet unborn, will contain a question something like this: What historical American of the nineteenth century has exerted the most powerful influence upon the destinies of his countrymen? And it is by no means impossible that the answer to that interrogatory may be thus written: 'Joseph Smith, the Mormon prophet.'" (A Marvelous Work and a Wonder, pp. 411-2)