In Search Store and Search keyword / ISBN, you can search by ISBN.
When you search by title, keyword(s) or advanced search, fragments of words and phrases included in product name / book title (and/or descriptions if such option is selected in advanced search) can be used. For example, make a search with "sho" and you will get products / books that have that fragment in their names / titles such as shoe, Photoshop. If you then in advanced search select "Search in product descriptions" then you will get more results (products that have that fragment in their descriptions).
The reason full text search is not working for a two letter word is "Search term minimum length is 3 characters". When we search two words with space each word will be taken as a separate string so the restriction will be applied for both strings. So just remove the words like "the" "of", 'and", "or" ... in your search.