Experts are trying to crack code used by last month's hijackers
The American television network ABC said the book was found in the possession of Kamel Daoudi, who was arrested in England after the terror attacks on the US last month and has since been extradited to France.
Mr Daoudi worked at a cybercafe. French intelligence officials reputedly believe his familiarity with the net could have helped the groups who carried out the 11 September attacks keep in touch.
Mr Debat said the information came from judicial sources who told him the notebook with Arabic writing "seemed to be a codebook".
A BBC correspondent in France, Malcolm Brabant, said that if information about the codebook is accurate, it could help the many computer experts hired by the FBI to pick out hidden messages in e-mails sent before the suicide attacks in New York and Washington.
"This code book is a major breakthrough in the investigation," Mr Debat told ABC.
He said French officials believe terrorists would have received their final instructions for the plot hidden in e-mail messages or even in pictures placed on the net.
The discovery of the codebook suggests that the hijackers and their cohorts may have used several techniques to communicate covertly.
Before now FBI officials have said that the hijackers used public web terminals and unencrypted e-mail messages to keep in touch.
The codebook may reveal that the seemingly innocent messages actually contained "trigger" phrases that helped co-ordinate the attacks.
French officials have even suggested that the hijackers could have been sending and receiving instructions hidden in music or pictures attached to e-mails or placed on the net.
The file concealing the message looks innocent, but those with the right software can extract text hidden within it.
This technique of hiding information is known as steganography
The codebook could reveal the identifiers the hijackers used to spot the files bearing hidden messages.
However, no evidence of hidden messages has been found in a survey of two million images carried out by US researcher Niels Provos from the University of Michigan.
Suicide missions
The code book was reportedly found during a round-up of Islamic militants who were suspected of planning to launch suicide missions against American targets in France, including the US embassy in Paris.
The FBI has discovered that three of the suspected hijackers, including the alleged ringleader, Mohammed Atta, turned up at a motel at Hollywood in south Florida in August and demanded 24-hour internet access.
Staff who went into the men's room said they had two laptops and a stack of CD-roms.
Experts believe the need to be constantly on-line suggested the hijackers were waiting for important messages.
On this occasion, the hotel was unable to provide the telephone connections the men needed, and they left in an angry mood, reportedly saying "you don't understand - we are here on a mission".