When Guillem Balague first began asking people in football about Pep Guardiola, they all made the some comment: "he's very strange".

Some couldn't articulate the quality that made Guardiola seem so unique, but others provided anecdotes. Jose Antonio Camacho, the former Spain coach, described him as a "mystical type, constantly analysing things", while colleagues in the Spain squad at the 1992 Barcelona Olympics talked of him always wearing black and being something of a loner. All were observing the same quality, though, Guardiola's obsessive nature.

If there is one single trait that explains his success as a player and a manager, but also the tortured psyche that he carries with him, it is this compulsiveness. By the end of Guardiola's time at Barcelona, he was physically and mentally exhausted, and was committed to the year-long sabbatical that he had promised his wife and children. Yet within one month, Guardiola was asking a friend if he should return to football.

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The restlessness is typical of an inquisitive mind, but the intensity and the single-minded approach are what separate Guardiola from many of his contemporaries. In writing his authorised biography, Another Way Of Winning, Balague reveals the extremes of Guardiola's nature. It is an in-depth, detailed and insightful study of a manager whose career so far has been exceptional, in terms of achievement but also the emotional costs that Guardiola had to bear. At one stage, Balague interviewed Guardiola while the manager's son sat in a corner reading a comic. Balague felt guilty, but was reassured by another member of staff that the only way Guardiola could spend time with his children was if they visited the training ground, because that's where he spent all of his time.

"I started scratching [beneath the surface] and I realised the extent of his obsession, and that there was a Yin-Yang thing about him, that he seemed so strong in many ways but he was also so weak," Balague said. "He's going to take that [intensity] everywhere. What does it take to be an elite player or manager? There is something in the genes, perhaps, that forces you to go further than anybody else would. You and me would stop halfway and say, 'this is not worth it, too many training sessions, too tiring', whatever, but they just keep going. It is something that all the players and managers recognise in Pep, but he multiplies it by 10. His brain is built that way. So it doesn't matter where he is, it will just be a short career."

Guardiola has been planning his return to the game throughout the seven months he has spent living in New York with his family. There have been a number of suitors. Guardiola, though, was adamant that he wanted his future decided by the end of next month, leading to yesterday's announcement that he has agreed to sign a three-year contract at Bayern Munich, whose coach Jupp Heynckes will retire in the summer.

Guardiola is an Anglophile and has always wanted to work in England, but despite the lure of London or Manchester, and the resources that might have been available to create another exciting team (and the former Barcelona executives now working at City), the Spaniard would not contemplate waiting until the summer. Despite all of the trophies and plaudits accumulated at Camp Nou, Guardiola is still haunted by self-doubt. The Barcelona squad was already full of exceptional players, including the world's best footballer in Lionel Messi, and Guardiola understands – to a painful degree – that he will be judged again in this second job, and by how his new team measures up to his first.

"He wants to start preparing for next season, he wants to start watching videos on his laptop, but is it the right timing?" Balague said. "Why now? Is desperation a help? There will be 100,000 reasons why he goes to one club and not to another, and they will change by the minute. That's just how his brain is. He needed to find a club that has enough funds to change if it needs changing, a club that already has a huge percentage of players who can play his methods and the style he's going to impose, and definitely a club that can win something straight away. He's going to go to a club to get rid of the doubt that he has in his own mind: can I do it somewhere else?"

Balague's book is a full portrait of the man, including all his faults. That kind of scrutiny might have put others off – particularly those whose egos are fed by the triumphalism that comes with success – but Guardiola granted his countryman full access. At the end of the process, he even declined a request to read one chapter over for his opinion, and pledged instead to buy a copy of the book. The closeness was revealing for Balague, but the reaction to the book, and the promotional tour he is in the midst of, exposed another reality.

"The interest here is unparalleled anywhere else in the world," he said. "It's like it's their league now, not ours, and Barcelona and Real Madrid are their clubs."

n Guillem Balague will discuss his book, Another Way Of Winning, and Spanish football at Walkabout, Renfield Street, Glasgow, on Friday, February 1, with Gaizka Mendieta and Peter Adam Smith