The account is believed to be worth six figures and was previously held by The Promotions Factory.
Frank has been charged with growing The Book People’s brand awareness, championing CSR initiatives and "encouraging the nation to fall back in love with reading".
Frank said The Book People is in "an exciting period of transformation", with a mission to ensure all children have access to books.
The business aims to do this by committing to "accessible pricing, investing in more school book buses and launching a new e-commerce platform".
Frank’s remit will see the agency develop and activate a series of "stand-out moments" across owned, earned and paid channels, with a focus on growing The Book People’s social and content offering.
The ultimate goal is to raise awareness of The Book People’s 'direct to schools’ business and launch its new CSR programme – The Big Book Boost, which commits the brand to donating more than a million books by 2022.
"The Book People is the biggest bookseller you’ve never heard of and we’re going to change that," Frank founder and managing partner Andrew Bloch said.
"It is an organisation with purpose at its heart and through its new CSR programme, will give back £1 to a school of their choice for every order over £10 – this is such a brilliant initiative and as a dad of two, I’m fully behind getting as many children to read as possible."
Claire Bayliss, The Book People CEO, said: "I’ve personally worked with Frank for a number of years, so I knew they were best placed to support across an ambitious brand transformation and help The Book People meet its business objectives.
"It’s our goal to inspire the nation to fall back in love with reading and move from being story sellers to storytellers. As an agency with creative flair and the ability to tell a great story, we’re confident in Frank as the right partner for us."