Digital books helped Amazon experience a bumper Christmas as global sales soared 22% to $21.27bn in its fourth quarter while physical book sales growth stalled.

However the online giant’s net income plunged 45% to $97m in its fourth quarter to December 31 as it continued to invest.

Amazon founder and chief executive Jeff Bezos said: “We’re now seeing the transition we’ve been expecting. After 5 years, eBooks is a multi-billion dollar category for us and growing fast – up approximately 70% last year.

“In contrast, our physical book sales experienced the lowest December growth rate in our 17 years as a book seller, up just 5%. We’re excited and very grateful to our customers for their response to Kindle and our ever expanding ecosystem and selection.”

Tablets were the etailer’s top-selling items for the second year in a row. At the year end, Kindle Fire HD, Kindle Fire, Kindle Paperwhite and Kindle held the top four spots in the online giant’s worldwide best seller charts.

Amazon’s digital media selection has grown to over 23 million movies, TV shows, songs, magazines, books, audiobooks, and popular apps and games in 2012, an increase from 19 million at year-end 2011.

For the year, Amazon made a net loss of $39m as it continued to invest in the business, while net sales soared 22% to $61.09bn.

The retailer expects sales to reach between $15bn and $16.6bn in its first quarter, up by between 14% to 26%. It expects to make between a $285m loss and $65m profit on an operating basis over the quarter against a $192m profit last year.