Are you asking for my personal opinion? Or is your instructor asking for your personal evaluation of Treasure Island? If your instructor wants to know what you think of the readability and interest level of the story, that means you need to form your own answer, not ask eNotes editors for their thoughts.
Readability and interest level - did you find the story interesting? Could you picture the scenery and/or action in your mind, based on what you were reading? Did you find it hard to stop reading at the end of a chapter because you couldn't wait to find out what happened next? If you answer these questions "yes," that indicates that you probably found the book to be very readable and highly interesting. If you found the book to be boring, couldn't get through reading more than a couple pages without having to remind yourself to keep plugging away, didn't identify at all with the characters in the book, and forgot what you read as quickly as possible, then you apparently found it to not be readable, enjoyable, or interesting.
My guess is that your instructor wants to know your evaluation, not mine!