Nicholas Evans was said to be stable in Aberdeen Royal
The author of the Horse Whisperer book was taken to hospital after picking and eating poisonous mushrooms while on holiday in Scotland.
Nicholas Evans and his wife Charlotte were in Moray when their party cooked and ate mushrooms picked in woodland.
The couple, from Totnes, Devon, were taken to Aberdeen Royal Infirmary along with Mrs Evans' brother Sir Alastair Gordon-Cumming, and his wife, Louisa.
An NHS Grampian spokesperson told BBC Scotland they were "stable".
The four were initially admitted to hospital in Elgin and then transferred to Aberdeen.
They were given dialysis and other forms of treatment to support their kidney functions, and were said to have responded well.
A family friend who visited them said all four were walking about and were in a cheerful and positive frame of mind.
They were all said to be "extremely grateful" for the rapid and effective intervention provided by the NHS.
Author recovers after being taken ill
Mr Evans' best known book, the Horse Whisperer, has sold more than 15m copies worldwide.
It tells the story of a talented trainer, with a particular talent for communicating with horses, who is hired to help an injured teenager and her horse back to health after an accident.
The story was made into a movie directed by and starring Robert Redford in 1998.