...I really enjoy playing the piano.

You can use "enjoy" + (verb+ing) to tell someone about things that you like doing. Please remember:

"...enjoy playing..." is correct;

"enjoy to play..." is WRONG.

Another example is:

I enjoy playing golf, but I don't like watching it.

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...I'm also very interested in history books.

This is used to talk about hobbies. Be careful: "interested in" and "interesting" sound similar, but actually have different meanings. "Interested in" is usually used with a noun, but can also be used with the -ing form of the verb.

Examples:

I'm interested in photography.

I'm interested in collecting antiques.

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How long have you been playing the piano?

"How long have you been" + present continuous tense is used to ask about situations which started in the past and are still going on now.

Examples:

How long have you been working for Kobe Steel?

How long have you been studying Japanese?

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Since I was about 7 or 8, I guess.

"Since" is used here to answer a question like "How long have you been (doing something)?" It tells us when a situation continuing now started in the past.

The following examples are answers to the above questions:

Since 1992.

Since last summer.

Since I entered university.

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