postgreSQL: how to duplicate a row

Specify id column only if you specify its value (and it's not your case). You want to use next sequence web_book_id_seq value, so do not specify it in your INSERT query.

Your INSERT should looks like this:

INSERT INTO web_book (page_count, year_published, file, image, display_on_hp, name, description, name_cs, name_en, description_cs, description_en)
SELECT page_count, year_published, file, image, display_on_hp, name, description, name_cs, name_en, description_cs, description_en
FROM web_book
WHERE id = 3;

You need to create a new ID for the newly inserted row:

INSERT INTO web_book( 
   id, page_count, year_published, file, image, 
   display_on_hp, name, description, name_cs, 
   name_en, description_cs, description_en
)
SELECT nextval('web_book_id_seq'), 
       page_count, 
       year_published, 
       file, 
       image, 
       display_on_hp, 
       name, 
       description, 
       name_cs, 
       name_en, 
       description_cs, 
       description_en 
FROM web_book WHERE id=3;

As mentioned by ClodoaldoNeto you can make things a bit easier by simply leaving out the ID column and let the default definition do its job:

INSERT INTO web_book( 
   page_count, year_published, file, image, 
   display_on_hp, name, description, name_cs, 
   name_en, description_cs, description_en
)
SELECT page_count, 
       year_published, 
       file, 
       image, 
       display_on_hp, 
       name, 
       description, 
       name_cs, 
       name_en, 
       description_cs, 
       description_en 
FROM web_book WHERE id=3;

In this case you don't need to know the sequence name (but it is a bit less obvious what's going on).