What are they? They are stories or oral histories created by someone else and put into book or manuscript form. I have a couple that mention my ancestors that were published at the bicentennial and one that was published at the city's centennial celebration. All mention my ancestors to some degree or other. I've listed them on the side bar under "Bookshelf". They are usually available for check out from a local library. In fact, all the books listed in my bookshelf have been checked out at the library. I haven't yet decided which book I want to buy, but I certainly don't need to buy them all.
Compiled histories are another good SECONDARY source. They should not be taken at face value. These are usually stories and remembrances and we all know how we like to embellish our stories and how our memories become skewed or sometimes fail altogether. Like the census, use the clues found in the stories to help you map your way to more research in the future. For example, in "A Century In the Foothills" there were stories from my grandfather, my great grandfather and my great grandfather's brother that all said, essentially the same thing. My great great grandfather came to the US after my great grandfather (his son) and settled in St. Paul MN. Since three people told the same story, I know that I should look for my great great grandfather in St. Paul and approximately the time frame that he was there. It also gave a clue to his death date, though I'll have some digging to do to verify exactly when that was. So, while not a primary source, the compiled record is certainly a wonderful secondary source full of information and clues and so much fun to pick through.
You can't just read and be done. You need to read and re-read and fill in your Family Group Sheet as you go along. Then create an abstract from the manuscript. An abstract is all the very pertinent information in the story, the who, what, when, where and why. What I've done is photocopy the title page, all the pages that mention my ancestors, any pages with photos of my ancestors and all the stoies written by my ancestors. Then I highlight all the bits I'll want for my abstract. I type up the abstract, fill in my family group sheets and cite my source. Now I know what the information is, where I got it and I can find it again or tell someone else how to find it, in the future.