We just released a new feature in our full-text Large Scale Searchin HathiTrust. When you do a search, you will see check boxes next to each search result. You can select items you want from the search results and create a personal collection. This should make it much easier to do repeated searches and explore a targeted subset of the HathiTrust volumes.
If you are not logged in, the collection will be temporary. If you log in, you can save the collection permanently. This enables users to do focused searching within a selected subset of search results.