reference books
helpful tools
dictionary
A dictionary is a book that list a words of a language in alphabetical order and gives there meaning, or gives the equivalent words in a different language . I personally would use it for a definition of a word.
encyclopedia
an encyclopedia is a set of books that tell you stuff about a subject. I would use to find out something for a project.
thesaurus
a thesaurus is a book that lists a words synonym. I would use it to look up a word