Started making illustrations for Albatros Publishing and won the Albatros Publishing Award for illustrations to "Hello, Is Anybody There?", a book by Jostein Gaarder in 1998; received the Golden Ribbon (in Brno) for illustrations to "The Enigma and the Mirror", a book by Jostein Gaarder in 1999; contributed illustrations to tens of books by 2004.

His best works include illustrations to "On the Motherland's Fathers' Czech Blood" and "On the Secret of the Queen of the Cows", books of newspaper columns by Michal Horáček (published by LN Publishing) that received rave reviews and became bestsellers.

Book illustrations have gained him many admirers abroad, too. For example, the "Irish Leprechaun Stories" by Bairbre McCarthy with Jelen's illustrations (published by Albatros) was a great success, and was followed by a version published in Ireland (in English). In 2009 the former Irish Prime Minister Brian Cowen gave a copy of this book to US President Barack Obama as a present for his daughters.