Search is important way for users to find information in help.
Search is not the most effective tool for finding information, but it is the tool users prefer to use. Many help systems omit indexes.
We used to find things, now we search.
What are the obstacles to find? Don't know keywords, can't ask questions, too many results, no synonyms, and more.
Google's predictive search as you type characters reminds of.... old-style Microsoft Help index. Search technology provides the best of using index in help. So used to predictive like in Google, when it doesn't happen elsewhere, think something;'s wrong.
Faceted search is kind of way of combining search and TOC. Doesn't require hierarchy that TOC imposes. Classify information, using metadata, by specific characteristics (facets). Facets can take on values. Users explore information by choosing required facets. Combine facets to narrow search. Results presented in any order. Can be used to refine results of full-text search.
Factors that turn search into find
Stop words, exclude specific words from search matching, marginally reduces file size and increases search speed.
Facility to exclude topics from search, so topics appears only in specific contexts.
Search results synopses, show first few words or key extract from search result, enables user to assess relevance of search result.
Phrase matching, phrases in quotes, usually unnecessary.
Search filtering and faceted search, follows concept of information types, commonly used on e-commerce sites, achievable with concept keywords and search filters.
Ranking of search results, results determined by number of occurrence of keywords (indexing automatically promotes topic in results), metadata.
Metadata is the key to flexible and effective search, enables you to avoid zero search results, especially with synonyms.
Predictive search, reduces keystrokes, provides better "scent of information," discourages "long tail" searches.
Practical ways to implement predictive search
Google Custom Search, use PHP and jQuery to add autocomplete, turnkey autosuggest technologies such as PredictAd.