Linksmartusa.com was in the Google search result
when entered either "linksmartusa" or something
like "cs-mount lenses". Suddenly, about a month ago,
Google search results doesn't show linksmartusa.com
in its search result anymore. The linksmartusa.com
web site has not been changed much in that period.

My questions are,
1) Why Google search could not locate
    the linksmartusa.com now?

2) How to keep linksmartusa.com site info
     in the Google search database continuously?

3) Why Google not providing its searching engine
    manual but asking people pay for search engine
    usage hints?

Google's Information for Webmasters has *always* been free:

Google Information for Webmasters
://www.google.com/webmasters/

It's easily found by going to the Google home page, and selecting the
Jobs, Press and Help link.

Google Answers does not exist solely to provide information to
webmasters.  We are here to help people accomplish all manner of
research tasks, either because they aren't skilled searchers or (more
frequently) because they just don't have the time to do the research
themselves.

--Missy