The Book by Alan Watts, one of his last, was published in 1966. A friend (perhaps a spiritual guide) recommended it to me around 1972, beginning my interest in Eastern philosophies.
The Book is very clear and concise, packed with interesting references to physics, biology, psychology and computer science, none of which feel dated today.
This year on my birthday another generous spiritual mentor gave me a new copy of The Book for my birthday, after I mentioned that I wanted to reread it and that my last copy had disintegrated (danged impermanence!).
I decided to write up a few notes to record my thoughts on rereading. That way if I feel the urge to revisit The Book in another half century I will be able to refer to my notes to refresh my memory.
Of course Watts himself intended The Book not as a sacred tomb, but as a point of departure for new experience:
So, kind teacher, wherever your spirit wanders, I confess that I have only partly followed instructions. I have not gone back again and again for hidden meanings. But The Book was so well and clearly written that it has remained in my consciousness all these years helping to acquire what little scraps wisdom I have been able.