The Buddhist library was built in regin of King Rama I, containing the Tipitaka, (The Buddhist scriptures) revised at Wat Mahathat, and after the revision was completed, transferred a new copy of it to house in this beautiful spired library,

There are statues of the giants and human-faced Naga guarding all of four doos.
Four stone Buddha images of Javanese style in the 14th or 15th century are placed
at the four corners of the external walls.

These models of the original ones which are now kept in the museum of the royal temple. The original Buddha images were made of volcanic stone and were presented
to King Rama V by governor of Java during the Royal visit to Indonesia.

Around the library, there are four small pillars Royal Pavilions on the Pillars contain the Payal symbols of all the kings to present dynasty. The mondop has been used by the King to enshrine objects of veneration. Phra Mondop is never open to
the public.

source : Tourist Guide to Thailand book,Thailand-sawasdee.blogspot.com