I will sound blasphemous, but I am honestly convinced: nothing. Despite the fact that reading books is so valued.
A modern book is a collection of letters. So is a magazine. A book can be hardcover or softcover, large or small, with many or few illustrations, written in a simple or complex way. A magazine is the same.
How is a book different from a magazine?
Here's one real difference: books have an International Book Serial Number (ISBN), whereas journals do not.
(Not counting, of course, that journals are recurrent - issued in numbers, monthly, yearly or over any other period.)
But who says that this set of letters is a book? ...The author himself.
Of course, the Lithuanian ISBN and ISMN (Music Publications) Agency, which follows the recommendations of the Lithuanian National Martynas Mažvydas Library, international standards, and the laws of the Republic of Lithuania, also draws attention. But, to my knowledge, there is no clear standard for distinguishing between a jumble of letters and a book. An electronic book in .pdf format can also be considered a book. A synopsis that is printed in ten copies can also be considered a book. A two-hundred-thousand-word giant can be considered a book, as can a hundred-word storybook...
So how is a book better than a magazine if the format is meaningless and we only value the words?