I won't tell you the whole story, since that would spoil the fun of you reading it.
I'll save that information for you, so that you'll enjoy reading the book.
ご質問ありがとうございます。
① "You'll find that out when you read it."＝「それは読んだら分かります。」
② "I won't tell you the whole story,"＝「話の全ては教えません、」
"since that would spoil the fun of you reading it."＝「あなたの読書の楽しみを奪ってしまうことになるので。」
③ "I'll save that information for you, "＝「その情報は言わないでおきます」
"so that you'll enjoy reading the book."＝「あなたが（その）本を楽しく読めるように。」
ご参考までに。
アメリカ合衆国 
