Two very different heroines are poised to return to the nation’s bookshelves: Tiffany Aching, teenage witch and protagonist of the late Terry Pratchett’s final novel, and Lisbeth Salander, the girl with the dragon tattoo dreamed up by the late Stieg Larsson.

Both Pratchett’s The Shepherd’s Crown and David Lagercrantz’s continuation of Larsson’s Millennium trilogy, The Girl in the Spider’s Web, are published on 27 August, and booksellers are bracing themselves for the influx of fans desperate to get their hands on the heavily embargoed titles. A midnight opening at Waterstones in Piccadilly in London will mark the release of The Shepherd’s Crown, with events planned up and down the UK. Pratchett, who died in March after being diagnosed with Alzheimer’s in 2007, completed the novel last summer. The 41st title in his bestselling Discworld series, it will be the fifth to feature Tiffany.

The Girl in the Spider’s Web UK cover

“We’re hugely, hugely excited,” said Kate McHale, teenage and science fiction buyer at Waterstones. “But it is going to be a bittersweet moment, as this is the last novel from one of British fiction’s great writers.” With titles including Harper Lee’s Go Set a Watchman and EL James’s Grey already released this summer, “it has been an incredible year for fiction, but this is certainly one of the biggest,” said McHale.

Independent publisher MacLehose Press, meanwhile, has a print run of a quarter of a million copies lined up in the UK for Lagercrantz’s follow-up to the Millennium trilogy, which kicked off with The Girl with the Dragon Tattoo and has sold more than 75m copies worldwide. The Girl in the Spider’s Web, which is authorised by Larsson’s father and brother but opposed by his partner Eva Gabrielsson, will see Salander attempting to hack into the US National Security Agency, and getting back in touch with journalist Mikael Blomkvist.

The contents of both novels have been a closely guarded secret, with even booksellers as yet unaware of what will befall Tiffany. “I have heard there is quite the event in this one, though,” said McHale.

“It’s definitely going to be one of the major releases of the year for us,” said Waterstones crime buyer Joseph Knobbs of the Lagercrantz novel. “We’re just waiting and seeing what the reaction will be to it – it’s been kept very closely under wraps.”

Publisher Transworld’s official description of Pratchett’s novel reveals that “an old enemy is gathering strength” and that “now Tiffany stands between the light and the dark, the good and the bad”. “As the fairy horde prepares for invasion, Tiffany must summon all the witches to stand with her. To protect the land. Her land. There will be a reckoning …” it says.

The Shepherd’s Crown UK cover

Pratchett’s Facebook page, in typically wry tones, says the novel will also include “a devilish smart goat named Mephistopheles, who can count to 20 and use the privy”, that it will “make you see the humble Garden Shed in a whole new light”, and that “an elderly gentleman’s toenails can be used as a powerful Feegle weapon” (the Nac Mac Feegles are six-inch fairies with red hair, blue skin and a strong Scots dialect).

“I rather like how I don’t know what to expect,” said journalist and fan Graeme Neill, who has been blogging about his reread of all Pratchett’s novels at the Pratchett Job. He added that the Tiffany Aching novels, a young adult series, were “somewhat under-read and under-appreciated by some Pratchett fans but I think they are among the best books of his later career”.

Neill said The Shepherd’s Crown would be “bittersweet to read as after that, there’s no more Discworld, although that is as it should be”. Pratchett’s daughter Rhianna Pratchett has said that neither she nor anyone else will continue to write the Discworld novels because “the books are sacred to dad”.

“The excitement around this book has been tremendous to see. His passing was deeply sad but it did show just how much Terry Pratchett and his novels meant to people. I grew up reading the Discworld books and I’m sure many were inspired to dig out their old paperbacks or revisit old favourites by way of a tribute,” said Neill. “I know some fans are intending to put off reading this book as long as possible, but he wrote the books to be read. I won’t be able to help myself and will gleefully tear through it.”

At books industry magazine the Bookseller, deputy editor Benedicte Page said that Thursday would “kick the autumn publishing programme off with a bang”.

“The launch of Terry Pratchett’s final book will be a hugely emotional moment for the trade as well as for all his fans, and there’s also a lot of excitement over David Lagercrantz’s follow-up to the Stieg Larsson Millennium trilogy – though that book’s publication is proving controversial in Larsson’s home country, Sweden,” said Page.

“Also out on the same day are new novels from Andrew Miller and William Boyd, and Jamie Oliver’s latest cookbook Everyday Super Food. It’s the first of several big days for publishing this autumn, the biggest of all set to be Super Thursday on 8 October.” More than 500 titles are published on 8 October, as the books world sets itself up for the Christmas season.

“This is a huge publishing moment for MacLehose Press and Quercus. It doesn’t get much bigger than the continuation of what has been a global publishing phenomenon,” said Hannah Robinson, publicity director at Quercus, MacLehose’s parent company. The worldwide print run for The Girl in the Spider’s Web is 2.7m copies.