who wrote the first book about king arthur
The familiar literary persona of Arthur began with Geoffrey of Monmouth's pseudo-historical "Historia Regum Britanniae"
King Arthur
The familiar literary persona of Arthur began with Geoffrey of Monmouth's pseudo-historical "Historia Regum Britanniae" ("History of the Kings of Britain"), written in the 1130s. 
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King Arthur [SEP] Medieval literary traditions
The familiar literary persona of Arthur began with Geoffrey of Monmouth's pseudo-historical "Historia Regum Britanniae" ("History of the Kings of Britain"), written in the 1130s. The textual sources for Arthur are usually divided into those written before Geoffrey's "Historia" (known as pre-Galfridian texts, from the Latin form of Geoffrey, "Galfridus") and those written afterwards, which could not avoid his influence (Galfridian, or post-Galfridian, texts).