A reference book is one you do not read from start to finish. You dip into it, look something up, and usually stay longer than you planned. This section holds encyclopedias, world fact files, illustrated books of knowledge and a few delightful oddities that fit no other shelf. If you are looking for general knowledge books for the home, a gift for a curious child or a handy fact book for quizzes, start here.
The internet answers the question you ask. A good reference book answers the questions you did not know you had. A child who opens an encyclopedia to look up volcanoes will end up reading about dinosaurs, planets and the deep sea on the way. That kind of wandering is how general knowledge is really built, and no search box can copy it.
GK books and fact files are useful from school quizzes right up to competitive exams. For school children, choose a well illustrated book with short entries. For older students, a compact world fact book with countries, capitals, dates and records is handy to keep on the study table. For exam focused reading, see Exam Preparation.
Our own note on building a small home library has more ideas.