The sequel, James Potter and the Hall of Elders' Crossing, charts the adventures of Harry's son, James, during his first term at his father's old alma mater, Hogwarts School of Witchcraft and Wizardry.
Fans of the series have been in a frenzy since a trailer for the book appeared on the elders-crossing.com website last week.
Some have posted messages that Rowling could be in on the act and that an eighth book is genuine, some dismiss it as a "cruel hoax" while others don't care and are just desperate for their latest Potter "fix".
The 360-page sequel is the work of George Lippert, 37, a digital artist from St Louis, Missouri, who wrote it for his wife and sons, who are avid Harry Potter fans.
Mr Lippert intends to release a chapter a day throughout December, free of charge, on his website.
The computer-generated movie trailer for the book features Harry-Potter style theme music, thunder and lightning sound effects and icons revealing hidden passwords and clues leading to taster extracts from the "new" book.
One chapter discusses the attempt to tighten up broomstick flying legislation for underage wizards and witches after some high spirited alcohol-induced behaviour celebrating a Scottish sporting victory in the heart of London.
An extract from the chapter Technomancy, History and Magical Policies, states: "After several incidents of careless flight, however, including the now infamous Soulcake Eve Fiasco (wherein several under-age wizards, while celebrating Scotland's quidditch cup victory, carelessly imbibed fire-whiskey and attempted a quidditch match in Trafalgar Square, the Ministry of Magic moved to officially regulate underage flying."
The last page has an illustration of young wizards struggling to control their broomsticks while startled Londoners gaze upwards. This broomstick flying breaks the rule whereby wizards under the age of 11 are allowed to fly if under parental control and if they do not make themselves visible to "non-magical persons".
Another chapter features pages from a magician's manual discussing the incendiary properties of Peruvian ballistic beans, which Peruvian wizards can transform into a sauce known as "el salsa grenado".
Since its launch, the site has attracted more than 40,000 visitors.
One fan wrote: "This may well be a hoax, but in all honesty I can see them perhaps thinking about it one day. After all, Hollywood has pretty much become Sequelville over the last couple of years and with HP being such a money grabber, I won't write it off as out of the realm of possibility just yet."
Christopher Little, Rowling's agent, has targeted any unauthorised works trading on the Harry Potter name. But experts specialising in copyright law say it may be less straightforward to prosecute Mr Lippert because young James Potter makes a fleeting appearance in Harry Potter and the Deathly Hallows.
Mahesh Madhavan, a specialist in intellectual property law at Strathclyde University said: "If an insubstantial character from a novel is taken and built up by another author in a new story, that can be a defence against copyright infringements."
Earlier this month Rowling threatened to take legal action against an American publisher who wanted to produce a 400-page Harry Potter lexicon, claiming it would infringe her intellectual property rights.
The eBay auction site has also been sued for selling unauthorised versions of the books.
Last week Mr Lippert said: "I am a little worried that people will be angry when they find out it's not Rowling.
"But I hope Harry Potter fans will enjoy reading it as much as I've enjoyed writing it."