What" is usually a question word. Examples: "What are you doing?" "What are these?" "What is your idea?" "What" can also be used with the meaning of "the thing or things". Here are some examples: "What you need is a good night sleep." (= The thing you need is a good night sleep.) "We don't know what to do." (= We don't know which things to do.) "You know what to do." (= You know which things to do.) "That" is used to describe and define the thing you are talking about. For example: "I want the pen." "Which pen?" "I want the pen that is on the desk." Here's another example: "I saw that girl." "Which girl?" "The girl that you invited to the party." Now, regarding your specific sentence: "The book what I need" would mean "the book which thing I need." This doesn't make sense, does it? On the other hand: "The book that I need" makes perfect sense, because you are giving a description of the book. - See more at: http://www.helping-you-learn-english.com/difference-between-what-and-that.html#sthash.GSOg7JPb.dpuf