Wikipedia spanish entry for GM Julio Granda says (Google translates):
Julio Granda does not study opening theory. He has stated that he has only read one chess book, the third volume of Roberto Grau's General Chess Treaty.
Is it true?
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After several searches I can only find one source in which Granda is quoted about Grau's books. It's this part:
"Yo tenía muchas limitaciones técnicas, así que me ganó con bastante facilidad, pero fue muy generoso porque le dijo a mi padre ‘Che, al pibe le falta Grau’. Mi padre, que no tenía mucho conocimiento de ajedrez en ese entonces, preguntó qué tenía que ver el almirante Grau con el ajedrez, pero en realidad Jorge Szmetan se refería a Roberto Grau, un maestro que dejó una obra muy importante que se llama ‘Tratado General del Ajedrez’. Tuvo el detalle de enviarme los cuatro tomos desde Argentina. Lo anecdótico es que en el correo se perdieron el segundo y el tercero, que son los más importantes, pero mi padre hizo un esfuerzo y pudo conseguirlos"
There he explains that a stronger player (Jorge Szmetan) suggested his father to study Grau's books. Then Granda explains that Jorge himself sent the books by mail, that the second and third ones got lost during the sending, but nevertheless Granda's father eventually got all of them. Then in the same link (and in other similar ones I've found) it explains that his father had him working during weekends on the Grau's books.
All the mentions I've found about "he only read the third tome of Grau" were not quoted and were almost identical to each other.
EDIT: here's another mention to Szmetan and Grau. He explains that his father made him study several books, including the ones by Grau. While he was studying chess his brothers were playing soccer, so he just developed sort of a hate for studying chess. He even mentions it was like a "torture" to him.
EDIT 2: Ok, this is it! At the very beginning of this interview Granda is specifically asked about the Graus and the books he's read.
First things first; he doesn't make any specific mention to the 3rd volume of the Graus (it's a 4 volumes collection).
Secondly, after getting the Graus via Szmeta, it was his father who, during weekends, read the Graus to Granda. So it seems like, literally, Granda didn't read (but listened to) any of the Graus. In any case he states in this interview that the Graus was his only theoretical training.
Also, in this part of the interview he explains that during a one month waiting for his next tournament, he bought a book (El mosaico ajedrecístico, probably this one written by Karpov and Gik) and fully read it without a board ("I was so bored"). Then he states that he's currently (the video is from 2017) studying seriously the game.
So after watching this video it seems like the only book training Granda received were the four volumes by Grau, read to him during weekends by his father when he was about 10 (the game vs Szmetan was in 1977, Granda was born in 1967). After that he only read "El mosaico ajedrecístico". So it seems fair to claim that the legend about "the third volume of the the Graus" seems to be just a legend, but on the other hand it seems like the four volumes by Grau were his only "book training" during all his career (at least until his temporal retirement in the 90s), what looks really like quite and achievement.
"Needs citation!" (xkcd)
That Wiki entry should be either disputed or given a reliable source. I googled a bit: Chicago Tribune article Sounds like from an interview, but I'm still not 100% satisfied. Another link, the danger is everyone is copypasting from everyone else. E.g. here would be the place I expect that it would be mentioned, but it isn't.