Jack Higgins started life, both literally as literarily, as Harry Patterson. His first novel was Sad Wind from the Sea, an adventure thriller set in Red China, which appeared from John Long in 1959 and which set the trend for many of his early books. Another recurrent theme surfaced in Cry of the hunter, which dealt with the IRA, to whom Higgins would return time and again.
Cry of the hunter is most interesting, though, for its main character, an ex-IRA gunman called Martin Fallon. In common with most thrillers, Higgins usually allows his protagonists, no matter how unlikely, to survive at the end of the book, but with Fallon he makes an exception. Not only is Fallon Killed in Cry of the hunter but , thirteen years later, the same character re-appears in A prayer for the dying, only to be killed off a second time.
In between these two unfortunate appearances, Martin Fallon stayed active, but the time as a pseudonym. In 1962, Higgins started a new series of books for a new Publisher, Abelard-Shuman, and published them under the name of Fallon, although by the fourth book there had been a parting of ways, and the last book appeared from John Long. All six books under that name feature a single character, Paul Chavasse, who worked for a secret branch of British Intelligence, known as "The Bureau" - a rather less cultured and more violent version of James Bond. Oddly enough, although five of the books have been reprinted, one (Year of the tiger) has never appeared in paperback, possibly because it's theme (About a futuristic spaceship drive) was felt to far-fetched.*)
Higgins was obviously fond of the names Fallon and Chavasse, as both also appear as peripheral characters in the 1964 novel Thunder at Noon, along with a character called Steiner, later the name of one of Higgins' most popular protagonists in The eagle has landed. Thunder at Noon itself is an oddity in the Higgins canon. Starting off as a fairly standard Western set in Mexico in 1930, it was later revised as Dillinger with the eponymous gangster translated, for no apparent reason, into the role of the main protagonist.
Meanwhile, Higgins' third thriller from John Long, The thousand faces of night in 1961, followed the same path as Cry of the hunter by introducing a protagonist, Hugh Marlowe, whose name Higgins was later to adopt as a pseudonym. Marlowe was the name that Higgins used for thrillers that he wrote for Abelard-Schuman that did not feature Paul Chavasse. There were three books published under this name, with no common theme, the first (Seven pillars of Hell) sharing the dubious distinction , with Year of the Tiger, of being the only Higgins thriller not to have been reprinted in paperback.**)

After two more thrillers from John Long , Higgins changed tack considerably in Pay the Devil, a historical novel set in poverty-stricken Ireland in 1865. This appeared from yet an other publisher, Barrie Rockcliff, and was published under the name "Harry Patterson", but sadly sank without trace and never appeared in paperback. The same fate was suffered the next year by Higgins' only other book from Barrie Rockcliff, a love story called A phoenix in the blood.
It was to be another 25 years before Higgins again strayed from the "safe" ground of thrillers with the publication of Memoirs of a dance hall Romeo - to the consternation of many critics who seemed ignorant of his previous sorties into the "mainstream". This volume did see paperback publication, but that probably confused many Higgins fans expecting yet another thriller.
Meanwhile, more thrillers were appearing from John Long under the name of Harry Patterson, and in 1965 he started his second "series" of book, this time featuring an ambittered detective-sergeant called Nick Miller. Miller, however, did not have the staying power of Chavasse and only lasted three books. The third of the Miller books (Hell is always today) introduced Miller's boss, George Mallory, again illustrating Higgins' tendency to reuse names he had taken a liking to - Chavasse's boss had been called Graham Mallory, an inspector Mallory had appeared in another early thriller (Hell is to crowded) and two different Neil Mallory's were protagonists in their own books (as an ex-SAS colonel in Wrath of the lion and as a free lance pilot in The last place god made.
It was in 1986 that the Jack Higgins name first appeared, again as the result of selling a book to a new publisher, in this case East of desolation to Hobber Stoughton. Although only three books were to be published first by Hobber Stoughton, their Coronet imprint was responsible for publishing the paperback editions of many of Higgins' earlier novels, all under the Jack Higgins name.
During this period, yet an other publisher, Macmillan, approached Higgins for a book (A game for heroes) which subsequently appeared under the last of Higgins' pseudonyms to date - James Graham. Only four, unrelated, thrillers were to appear from Macmillan, all under the Graham name, although again many of the other books appeared in paperback from Pan, who subsequently became part of the Macmillan group.

I some ways, 1971 could be seen as a watershed for Higgins. June saw the publication of the last book to appear under John Long imprint, which had been responsible for the most Higgins books published. Then only two months later, August saw the publication of his first book from his sixth publisher, William Collins, who were to take him to super-stardom in the following years, publishing a total (to date) of 14 titles.
As Hobber stoughton had, by now, left the scene, the Jack Higgins name was retained from for the books appearing from William Collins, and the first three titles were much the same formula as before. However in 1975 came the book that was to make Higgins a household name - The Eagle has landed about a plot to kill Churchill during the World War II. With a much more detailed, and complex, plot than many of his books, it attracted a much wider audience, particularly after the successful 1977 film of the book with Michael Caine.
The Eagle has landed also introduced one of Higgins' most popular characters in the form of IRA gunman Liam Devlin, who had a secondary role in that book, but who later appeared as protagonist in both Touch of the devil and Confessional. The latter also featured featured a guest appearance by SAS major Tony Villiers, who had been protagonist of the previous book, Exocet.
The drawing power of the Higgins name was not immediately apparent and, when Higgins published three titles for his seventh publisher, Hutchinson, they appeared under his "real" name of Harry Patterson. However, subsequently paperback editions were careful to make it clear that this was "the author of The eagle has landed" and, despite two further changes of publisher, first to Heinemann and then to Chapmans, there have been no further pseudonyms and it seems likely that all future books will be "By Jack Higgins".

*) Note that Year of the tiger has been republished in 1996.
**) Note that Seven pillars of Hell has been republished under the name Sheba.