be an eligible prosecutor.
That's a goal not a purpose. It doesn't explain why you are reading the book now. It's a nice long term student goal but not a motivating purpose for a book.
Look at purpose as the journey toward your goal. Your goal is a point on a map marked X. There are many routes toward your goal and on your way to that goal you need to collect a few items (of knowledge, understanding and abilites). You'll select a route to collect the first item, then plan (your purpose) for the next item. Your purpose has some stops along the way toward a higher goal. Purpose is path towards sub goals that lead to your main goal. Your purpose says which way you're going because your intention is to collect what you need on the way to your goal.
Which way I'm going ... I'm reading this book [title] to learn (item you want to collect) so that I can [how will this knowledge benefit you?] e.g pass the exam so I can continue with the next level or study. So that I can work in an office. So that I can teach / explain it to... So that I can write about it. So that I can use it in my career. So that I can... [unlimited reasons]
Alex
