This link tries to go to http://nltk.org/book_1ed/, which returns an HTTP Status: 404 (not found).
When changed to use https://www.nltk.org/book_1ed/ or http://www.nltk.org/book_1ed/ it works.
Probably the automatic redirection stopped working (with non-www URLs).
Further, as it is likely related, going directly to http://nltk.org/book is also not being redirected to https://www.nltk.org/book/. Resulting in an HTTP Status: 404 (not found) when attempting. But http://www.nltk.org/book is being redirected to https://www.nltk.org/book/.
On the book's homepage, there is a reference link:
This link tries to go to
http://nltk.org/book_1ed/, which returns anHTTP Status: 404 (not found). When changed to usehttps://www.nltk.org/book_1ed/orhttp://www.nltk.org/book_1ed/it works.Probably the automatic redirection stopped working (with non-www URLs).
Further, as it is likely related, going directly to
http://nltk.org/bookis also not being redirected tohttps://www.nltk.org/book/. Resulting in anHTTP Status: 404 (not found)when attempting. Buthttp://www.nltk.org/bookis being redirected tohttps://www.nltk.org/book/.