"Happy reading!" - Korean doesn't seem to have a phrase with the same meaning and as short.
(즐거운 독서(를) 하세요 would be the direct translation, but it doesn't sound very natural)
I can think of a few different ways of saying it.
- 기분 좋게 독서를 즐기세요. Have a pleasant time reading.
- 즐거운 독서에 빠져 보세요. Immerse yourself in happy reading.
- 책 읽는 즐거움을 만끽하세요. Pamper yourself with the pleasure of reading.
* 만끽하다 = take a full enjoyment of something.