How to use custom tables in Views
There's a hook for exposing custom fields to Views: hook_views_data()


  Describe table structure to Views.
  
  This hook should be placed in MODULENAME.views.inc and it will be auto-loaded. This must either be in the same directory as the .module file or in a subdirectory named 'includes'.
  
  The full documentation for this hook is in the advanced help. 


The link also includes example code.