"This book is short."
Translation:यह किताब छोटी है।
[Repeating my reply to you from another thread in case someone else stumbles by this thread in the future]
You use होती when you are making generalizations or talking about something that is universally true. For example, when you say किताबें छोटी होती हैं, you mean that all books are short or that books, in general, are short. Without the होती, you would be talking about a particular set of books.