He's not even a saint in the RCC if I recall correctly though so his work is not really authoritative for anyone.
It certainly has been a hugely influential book over the last 600 years, being published in at least 1,000 editions. John Paul II was said to have had it in his hands when he died. It's part of the Ignatian Spiritual Exercises and is one of those Catholic books that Protestants often read.
It is true that he has not been canonized. I read somewhere that his cause was opened, but when he was exhumed, they discovered scratches all over the roof of the coffin and clumps of his hair in his hands. I guess he had been buried alive accidentally, and the powers that be assumed that he despaired, something saints aren't supposed to do.
It sounds awfully apocryphal to me, and I doubt we need a fantastic story to explain it. It took 400 years for St. Thomas More to be canonized, after all.
I think the book is worth reading.