The Treachery of the Blue Books or Treason of the Blue Books (Welsh: Brad y Llyfrau Gleision) was the name given in Wales to the Reports of the commissioners of enquiry into the state of education in Wales published in 1847. The term Brad y Llyfrau Gleision was coined by the author Robert Jones Derfel in response to the Reports' publication.
The public inquiry was carried out as a result of pressure from William Williams, Radical MP for Coventry, who was himself a Welshman by birth and was concerned about the state of education in Wales. The enquiry was carried out by three English commissioners, R. R. W. Lingen, Jellynger C. Symons and H. R. Vaughan Johnson. The commissioners visited every part of Wales during 1846, collecting evidence and statistics. However, they spoke no Welsh and relied on information from witnesses, many of them Anglican clergymen at a time when Wales was a stronghold of nonconformism.
The work was completed by 3 April 1847, and Lingen presented his report to the Government on 1 July of that year in three large blue-covered volumes ("blue books" being a widely used term for all kinds of parliamentary reports). The report was detailed. It concluded that schools in Wales were extremely inadequate, often with teachers speaking only English and using only English textbooks in areas where the children spoke only Welsh, and that Welsh-speakers had to rely on the Nonconformist Sunday Schools to acquire literacy. But it also concluded that the Welsh were ignorant, lazy and immoral, and that among the causes of this were the use of the Welsh language and nonconformity. This resulted in a furious reaction in Wales, led by Robert Jones Derfel, a bard, whose book-length response, Brad y llyfrau gleision was published in 1854 by I. Clarke in Ruthin; it had no immediate political consequences, although it was instrumental in the birth of the modern Welsh self-government movement. A measure of the anger aroused by the report in Wales is the subtitle Brad y Llyfrau Gleision. It is a reference to the infamous "Treachery of the Long Knives" when, according to Nennius and Geoffrey of Monmouth, the Saxons began their campaign of conquest against the native Britons.