THE SPANISH WAR An American Epic 1898. By G. J. A. O'Toole. 447 pp. New York: W. W. Norton & Company. $19.95.

G.J. A. O'TOOLE is a prolific writer, having produced six books in the last decade as well as many magazines articles. His interests range from a spy thriller to attempts to unravel alleged conspiracies surrounding the Lincoln and Kennedy assassinations. In fact, historical espionage seems to be a major concern, and his last book in this genre, ''Poor Richard's Game,'' should have made more of a splash. Using a fictional intelligence man's report, supposedly published in Philadelphia in 1836, Mr. O'Toole made a case that Benjamin Franklin was a double agent in Paris working for George III, as well as a member of a cult that worshiped Satan and engaged in sexual orgies (Franklin was in his early 70's at the time). He calls it ''an annotated novel,'' although ''fictionalized history'' might be a more appropriate description. He has a marvelous wit, and any reader with a sense of humor could enjoy this perfectly outrageous romp at Franklin's expense.

My problem is that I am not always sure that Mr. O'Toole really distinguishes between fact and fantasy, or if he actually believes that fiction is merely undocumented reality. Col. Michael Croft, whom Mr. O'Toole identifies as an ''amateur historian, professional detective, and a former Army intelligence officer,'' states in a foreword to the Franklin caper: ''Much of this is true. Maybe all of it. . . . The final answer is probably locked up in some dusty but well guarded room in Whitehall where Her Majesty's government keeps the archives of the British Secret Service.''

Now, Mr. O'Toole has turned to straight history in this account of the Spanish-American War by which the United States finally dislodged Spain from our hemisphere and also acquired the Philippines. My initial skepticism over his approach to history was not assuaged when I read in his introduction that ''Almost unconsciously, the United States had taken the road to empire, and there was no turning back, not even had some prophet foreseen that Pearl Harbor, the Bay of Pigs and Vietnam had been made inevitable.'' Few historians would subscribe to such historical determinism - at least not, let us hope, in the 20th century. To my pleasant surprise, however, I discovered the book to be a lively and dramatic account of this country's path to empire, which can be highly recommended for entertaining, if not enlightening, reading. Even those sections in which the author seems to throw his note cards at the reader, with little effort at transitional passages, can be read as exciting intelligence reports.

Since Mr. O'Toole does not shy away from the term ''amateur historian'' in describing Colonel Croft, let me make clear that ''The Spanish War'' is amateur history, and I do not always use that modifier pejoratively. Indeed, I would be the first to admit that some amateurs write better history than do many, if not most, professionals. But professionals adhere to a set of values and accepted techniques that are enforced by a fraternity of scholars who continually judge each other's work. Free of this restraint, amateurs can employ techniques that enliven the reader's interest and enhance the dramatic effect of history. For the reader, this often involves inferring the mood changes, or of reading the inner thoughts of principal actors, which cannot be documented by professional standards, and has to be a highly speculative procedure. Thus, Mr. O'Toole sees in one of Theodore Roosevelt's speeches ''a rebuke to that small, dismaying voice within himself whispering that the historian 'Brooks Adams may be right' '' in rejecting the notion that an individual ''can influence the destiny of a race for good or evil.''

Mr. O'Toole also sees intrigue on the flimsiest of evidence. When Secretary of the Navy John Long insisted that there was no ulterior motive behind sending the U.S.S. Maine to Havana and recorded in his diary that ''this happens to be the truth, the whole truth and nothing but the truth,'' Mr. O'Toole observes that this ''certainly seems excessive reassurance for someone to offer himself concerning his own veracity,'' and concludes that ''Long may have written this account without the complete candor one usually expects of diarists.'' Unless this conclusion was meant to be sarcastic, it sounds dreadfully naive.