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This year’s Waterstones children’s book prize winner features a green blob tucking its child – a smaller green blob – into bed.

Sleep Tight, Disgusting Blob by Huw Aaron is narrated by the parent blob who, through the course of the book, tells the child blob about all the other creatures also getting ready to sleep, including a yeti, Medusa and a Minotaur.

Aaron, a Welsh author and illustrator, was announced as the winner of the £5,000 prize at a ceremony on Thursday evening. His book topped the illustrated books category before going on to be named overall winner.

Booksellers, who vote for the prize, praised Huw’s “dreamy, painterly style and sweet rhyming text”, said Bea Carvalho, head of books at Waterstones.

Aaron is from Swansea, and now lives in Cardiff. Growing up, he spent much time doodling and writing stories, but trained as an accountant. “It shortly became apparent that drawing monsters and developing elaborate toilet-based puns were not the primary skills required of an NHS financial manager and, in 2009, Huw was belched out like Jonah of old from the whale of public-sector accountancy on to the beach of self-employment as a freelance cartoonist,” he writes on his website. He has created cartoons for magazines including Private Eye and Prospect.

Another book of Aaron’s, Unfairies, was nominated in the younger readers category, marking the first time in the award’s history that an author has been shortlisted for two titles simultaneously.

Ultimately, Janeen Hayat went on to win the younger readers category, with her book Evie and Maryam’s Family Tree, in which two classmates discover they have a family connection dating back to the partition of India.

A Language of Dragons by SF Williamson was named the older readers winner. The book is set in an alternate Bletchley Park, featuring dragons, where the protagonist, Vivien Featherswallow, has been recruited as a codebreaker.

The Waterstones children’s prize was first awarded in 2005. Past recipients of the overall award include The Hate U Give by Angie Thomas, Rooftoppers by Katherine Rundell and Blown Away by Rob Biddulph. Last year’s prize was awarded to Mikey Please for The Cafe at the Edge of the Woods. Along with the cash prize, the winner is promised “ongoing commitment” to their writing and illustrating career.

• Sleep Tight, Disgusting Blob by Huw Aaron is published by Penguin Random House Children’s UK. To support the Guardian, order your copy at guardianbookshop.com. Delivery charges may apply.