The Merlin Trilogy by Mary Stewart?

Reviews and things call it 'The Merlin Trilogy' and my mum read them as a trilogy when she was in her teens. Yet she has written 5 to do with King Arthur, merlin etc. Wikipedia calls it the Merlin series. these are them:


The Crystal Cave (1970)


The Hollow Hills (1973)


The Last Enchantment (1979)


The Wicked Day (1983)


The Prince and the Pilgrim (1995)





Is it a Merlin trilogy or do these last two books not contribute much to the story?The Merlin Trilogy by Mary Stewart?
The series is a quartet of books plus one extra, not a trilogy.





The extra book “The Prince and the Pilgrim” could be inserted between “The Last Enchantment”' and “The Wicked Day” if you want to consider it to be part of the same world as the rest of the series and want to read the series following internal chronological order.





Any reviewers who call this series a trilology are just displaying their ignorance.The Merlin Trilogy by Mary Stewart?
Your mother may have indeed read the trilogy, but a gap of only 4 years between books -- 1979 - 1983 -- leads me to believe that the the fourth book could have a great deal to do with the previous three. It's not impossible that a twelve year span could still keep the fifth book related to the previous four. John Updike's Rabbit novels are all about 11 years apart in publication.