By the end of his life, Samuel Langhorne Clemens had achieved fame as a steamboat pilot on the Mississippi, a globe-trotting lecturer and, of course, the literary genius who wrote “The Adventures of Huckleberry Finn” and other works under the name Mark Twain.

He was less well-known, but no less talented, as a literary critic. Proof of it has resided, mostly unnoticed, in a small library in Redding, Conn., where hundreds of his personal books have sat in obscurity for 100 years. They are filled with notes in his own cramped, scratchy handwriting. Irrepressible when he spotted something he did not like, but also impatient with good books that he thought could be better, he was often savage in his commentary.

“The English of this book is incorrect & slovenly & its diction, as a rule, barren of distinction,” Twain scribbled in his copy of a 1906 autobiography of Lew Wallace, the Civil War general who wrote “Ben-Hur.”

Now, for the first time, this little-known side of Twain’s life, plumbed occasionally by scholars, can finally be appreciated by his fans. In honor of the centennial of his death on April 21, the library granted The New York Times permission to examine this trove of books and record notes and markings Twain left behind in their margins.