BOSH!'s debut cookbook is number one in TheSunday Times' chart for the fourth week running.
After winning praise from celebrities and influencers, it shot to the top of the charts - where it has stayed for a month.
Discussing the success on Instagram stories, Theasby said: "Here's the Culture magazine from The Sunday Times...here's the book section.
"There we are. BOSH! Last week's number one - this week's number one. Boom!"
Firth added: "Vegan at the top!"
The pair shared their experience of producing the book on their YouTube channel
Speaking to Plant Based News about the success of the book when it launched, BOSH! said: "We're excited and honored by the overwhelmingly positive response to our first cookbook.
"That this was the fastest-selling cookbook of 2018 (not just the biggest vegan cookbook) shows just how much perceptions are changing.
"It now feels archaic to describe vegan food as dull and boring. So many great people are doing great work to make vegan food mainstream."
BOSH!: Simple Recipes. Amazing Food. All Plants. is available now. You canorder it here