PRESIDENTIAL libraries have been alternately called monuments to Pharaohs and treasure-troves of history. Today, whether museum or memorial, the nation's 11th presidential center will open on a hilltop here - inaugurated by perhaps the first gathering of five chief executives in history.President Bush and former Presidents Nixon, Ford, and Carter are scheduled to join the man of the moment, Ronald Reagan, to dedicate the Ronald Reagan Presidential Library and Center for Public Affairs in this community of Spanish-roofed homes one hour north of Los Angeles. It is an apt setting for a paean to the actor-turned-40th president. The mission-style complex looks out on a vista of elephant-hide hills that were once the backdrop for Hollywood westerns. Although Mr. Reagan never donned chaps here, Gene Autry, the Lone Ranger, and dozens of other celluloid cowboys did. This is also Republican-white-and-blue territory. None of the opposition that surfaced at Stanford University, where "left wing" faculty members objected to the campus as a library site, is evident here, even though this is the first library without ties to a president's hometown or a college. "This is Reagan country," says Charles Jelloian, director of the Ronald Reagan Presidential Foundation. The adobe-tiled complex on 100 acres of land houses a museum chronicling Reagan's life and presidency, a policy center, and storage for 54 million White House papers in acid-proof boxes - the most of any presidential library. Some 6.3 million documents will be released immediately, none of which is expected to be headline-grabbing material - mainly greeting cards and routine policy papers. By law, the most sensitive documents, including those dealing with the Iran-contra scandal, won't be made available for at least 12 years. Dr. Ralph Bledsoe, library director, promises no shading of history in releasing materials. In the museum, the president's life is presented in films, interactive videos, and exhibitions, as are the forces and ferment that shaped his ideology and tenure. Cases feature his boyhood in Illinois, photos from his Hollywood days, college essays, and a high-school yearbook in which he is listed as "Donald" Reagan (an apparent misprint his estranged former chief of staff, Donald Regan, might not appreciate). One gallery features economic initiatives - what detractors called "Reaganomics and another foreign policy. Some of the 75,000 gifts the Reagans received are on display, including a gold purse from the Saudis and a pistol from the king and queen of Spain. Still in storage: the surfboard from Pepperdine University and saddles from almost everyone. Dignitaries, former Reagan aides, and friends will be on hand to dedicate the privately funded $57 million complex. Jimmy Carter, the lone Democrat among surviving presidents, will attend if back from Africa. One who won't be there is Oliver North. He will likely be out promoting his new book, the one that charges Reagan knew everything about Iran-contra.