When building a bookcase, make sure that the shelves are no longer than 36 inches, 32 inches if the books are heavy.
In a basic freestanding bookcase, the
shelves fit into dadoes in the uprights. A dado is a cut across
the face of a board, in which the shelves will be inserted. This
method of joint provides greater support and a cleaner appearance.
The top shelf should be put in place with rabbet joints. Glue
can be used to reinforce all the joints.
-- Tips courtesy of HomeAdvisor.com