"On top of that, we get a couple of boxes of books a week from publishers and authors sending their stuff to us directly," she adds, citing recent contributions from the likes of local heroes Benjamin Law and Maxine Beneba Clarke.

"We've had about 100 people requesting to be 'book ninjas', which is people asking for stickers to put on their own books," says Kalus.

With added interest from local publishers, there are now "over 300 books" circulating the city loop, specially labelled with stickers instructing commuters to "take, read and return" them - and even review and chat about the titles on social media , like some kind of roving anonymous book club.

The idea, sparked by Berg's time working on London's similar Books On The Underground project, was originally launched in April with deliberately mislaid copies of The Great Gatsby and Paullina Simons' The Bronze Horseman, sourced from the duo's own bookshelves and a secondhand bookshop in Windsor.

While they've been flirting with the idea of making their deviousness official, by partnering with sponsors and potentially even Public Transport Victoria ("At this stage it's all handled by ourselves, and creating and distributing the stickers is quite a costly endeavour," says Kalus), the clandestine nature of the project's worked in their favour so far.

"Sometimes we like to stay on the train, put the book down, and walk down further along the carriage and just spy from the other end," Kalus says.

"You usually see people looking at it like, 'Gosh, what's that doing there?'. Or people often chase us going, 'Hey, you've forgotten your book!'.

"But we've seen a few successful captures. Most times it just gets picked up and someone takes it off the train with them, which is really great to see."

"We want people to follow the book's own journey and adventure, and it's really special seeing the way people are connecting with the books and each other."