He's not in the index

If you pick up a book and look for your ancestor's name in the index and don't find it you naturally assume there's no information in that book which can help you with your genealogy research. But as we say in Texas "hold on thar pardner". Keep in mind these tips provided by James Hansen, a reference librarian of the State Hisorical Society of Wisconsin.
1. An index is only an index. It is not a substitute for the record being indexed.
2. The larger the sizes of the index, the more easily pertinent listings are overlooked.
3. In a given record, any vowel may at any point be substituted for any other vowel.
4. Virtually every pre-WWII record, in whatever form we see it today, originated as an attempt by an individual to put on paper what he or she thought was heard.
5. There is no perfect indexing system.
6. It doesn't matter how you spell your name; it only matters how the indexer spelled it.
7. Just because an index is described as complete or comprehensive, doesn't mean it is complete or comprehensive.
8. It you haven't found it in an index, you can only conclude that you haven't found it in an index. You cannot conclude that it's not in the record.
9. The index isn't always at the back of the book.
10. Sometimes it is best to ignore the index all together.

Happy researching.