Is Michael Crichton, the father of the techno-thriller your favorite writer? Have you read everything that he has written? Are you sure?
Like many authors, Michael Crichton has written under a pseudonym or pen name. Crichton has written under the pen names of Jeffery Hudson, John Lange, and Michael Douglas. As Jeffery Hudson he wrote "A Case of Need" for which he won the Edgar Award for the Best Mystery of the Year in 1968, and as John Lange he wrote a total of eight spy thrillers.
Why does a writer choose to write under a pen name? The answers are as varied as the writers themselves. Some choose to do so simply to protect their privacy. Others use a pen name when writing outside the genre that they are famous for. For example Agatha Christie wrote romances under the name Mary Westmacott and Dean Koontz has written under more than 12 pen names, including the name Jane Nichols.
What does this mean for the reader? On the plus side it means that there may be more books written by your favorite author than you thought, although you might have to do a little hunting to find out what they are. This can be done by browsing an author's biography. For instance, a biography of John Le Carré might mention that his real name is David John Moore Cornwell. Or you might peruse one of the many websites that list pseudonyms, which can be found easily via your favorite search engine.
On the negative side, it is not unusual for a book written under a pen name to be reissued under the real name of the author. For example Richard Bachman wrote the novels, Thinner, Rage, The Long Walk, Roadwork, and The Running Man. They have all since been reissued under the author's real name - Stephen King. This practice of reissuing books under the authors real name, has the danger of causing readers to repurchase books that they already own. A plight which can be further exacerbated when a publishing house changes the title, as well as the author's name!