I found a disturbing bookplate in a Widener book. It said: "This book was stolen from Harvard College Library. It was later recovered. The thief was sentenced to two years at hard labor. 1932." Isn't that excessive punishment?

While the statement is literally true, it is also misleading. The thief stole a total of 2,504 books in the 1920s from Widener.  You can read the whole story in the Crimson article of November 4, 1931.  Christopher Reed writes about the theft and the bookplates in the March-April, 1997 issue of Harvard Magazine.  This is from his article Biblioklepts:

 In 1931 Joel C. Williams, A.M. '09, Ed.M. '29, a former instructor at Groton and a former high-school principal, was caught with 2,504 stolen Widener books at his home in Dedham, Massachusetts. He said he was preparing himself for a college professorship. His thieving had begun eight or 10 years before, but had stopped a year and a half before he was caught when, according to a newspaper account, "extraordinary steps were taken by the Harvard authorities to prevent students 'sneaking' books out of the library without permission. A turnstile was erected at that time and suspicious bundles were ordered examined." An editorial writer in the Boston Post said that the case "suggests impaired mentality." When the books came back to Widener, librarians had an acerbic bookplate printed and affixed to each volume. It reads, "This book was stolen from Harvard College Library. It was later recovered. The thief was sentenced to two years at hard labor. 1932." A security measure of sorts.