The prosecution said Abdul Patel was a "radicalised" Muslim and his possession of the manual was "not innocent."
Mr Patel, of Clapton, east London denies having an article for terrorism and having a document which would assist someone for use in terrorism.
Officers found the manual at his wife's former family home, the court heard.
Mr Patel was aged 17 at the time of the discovery last August.
'Not innocent'
The manual was meant for American bomb disposal officers and gives step-by-step directions to produce a "viable, improvised explosive devices - home-made bombs", prosecuting Peter Wright QC said.
He told the jury: "Abdul Patel is part of a web of radicalised young Muslim men in which his possession of the explosives manual was not innocent.
"It was entirely deliberate. It was available for use if called upon.
"It was in the custody of a young man who was ready, willing and able to assist in a cause he believed in," he added.
The court heard officers found two boxes at his wife's former family home which were marked, "Don't Touch".
They contained the manual, pictures of al-Qaeda leader Osama bin Laden giving a speech on the 9/11 attacks, pictures of the attack and some of the terrorists who participated and pictures of suicide bombers and beheadings.
'Catastrophic consequences'
Officers also discovered part of a stun gun, night vision binoculars and correspondence in the boxes and a CD containing Islamist speech aimed at American service personnel.
Mr Patel said the boxes were left in his care by a friend.
Mr Wright told the jury that the man in question is an Islamist who was in touch with Mr Patel over phone and entered the UK using a fake passport.
He added the two exchanged text messages which "illustrates the close and surreptitious" relationship between them and other Islamists which "place the finding of the manual in its correct context".
Mr Wright said the manual was kept "for a terrorist purpose should the need arise" and could have "catastrophic consequences".
The trial will continue on Monday.
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