When a textbook author can't define something, he should NOT try to write about it. This author doesn't know what the past perfect is so he doesn't define it and only gives a garbled explanation of what he thinks it might be and struggles to make up sentences that might, if he gets lucky, illustrate it correctly. If you are trying to learn English grammar, let this one go. The past perfect is an important tense and if you want to speak and write English correctly, look elsewhere.
He makes an equal mess of the present perfect. Buyers beware. Students -- particularly students -- beware. This text contains incorrect information, in the regard ONLY of these two tenses. There may be other correct information in this book but it does not show up in regard to the present perfect and the past perfect. Even if he speaks ENGLISH English, it's wrong. He can't make it make sense. He grasps at straws. If you care, and want explanation, send me an email. If you don't care -- if ignorance suits you -- please stay away.
Author Eastwood. If this bothers you, tell us what it is, i.e, DEFINE it. Imagine you're writing an entry for a dictionary. "The Past Perfect is ..." (Oxford University Press, b.t.w, won't help you. They don't know either.)
Note that it is NOT 'had + the past participle'. That is the FORM it takes but that does not define it.