In English, "He has books" means that he has some books without saying what books he has, while "He has the books" refers to a specific group of books that is somehow clear from context -- for example, because you have mentioned that particular group of books before already.
The situation is similar in German with the distinction between Er hat Bücher (general) versus Er hat die Bücher (specific).
This sentence is Er hat die Bücher, so it should be translated to "He has the books."
"He has books" may be a fine English sentence but it's not a good translation of Er hat die Bücher.
Tips and Notes from the "The" lesson: https://www.duolingo.com/skill/de/The/tips-and-notes
Complicated by the "Accusative" lesson: https://www.duolingo.com/skill/de/Accusative-Case/tips-and-notes
Master these before moving on because there are more "the" forms to come!
bucher is not a German word; it is spelled Bücher with capital B and ü. (If you can't make an ü, write Buecher.)
Sie hat Bücher. = She has books.
Er hat die Bücher. = He has the books.
Both are grammatically correct sentences. The first one says that she has some books (but it's not important which ones), while the second one says that he has a particular collection of books that the listener can identify.
It's "books" versus "the books".
It changes from das to die bucher because it is in plural or because bucher is accusative? or both?
Also, it's Bücher with capital B (it's a noun!) and with ü, not u (Bucher would mean "booker -- someone who books [e.g. trips]"). If you can't make the ü, write ue instead: Buecher.
All nouns take die in the plural, in the nominative and accusative cases.
der Mann - die Männer; die Frau - die Frauen; das Kind - die Kinder (nominative)
den Mann - die Männer; die Frau - die Frauen; das Kind - die Kinder (accusative)
you mean i will attach "die" with any plural either it is masculine, neuter or feminine?
Yes. There are no gender distinctions in the plural in German. There's no "masculine plural" or "feminine plural" or "neuter plural" -- just "plural" which is the same for all nouns.
For example, der Mann (masculine) - die Männer (plural); die Frau (feminine) - die Frauen (plural); das Kind (neuter) - die Kinder (plural).
You could almost consider "plural" a gender of its own, alongside masculine, feminine, and neuter.