Even after a comprehensive study of Islam and different philosophical schools, enough to make him a famous scholar, Abu-Hamid Al-Ghazali was somehow not fully satisfied. It was not an easy journey for him until his search ended with "satisfaction in the way of Sufism", as he wrote in Al-Munkiz. When he realized that, he was at the peak of his career, yet he felt he had to have time for himself. At 38 he left a prestigious teaching job and stayed away from social life for eleven years. During that period he was seen several times. He would get away whenever people recognize him. He was quite content with what he had found in Sufism, and had no intention to teach anymore. He needed his time alone, yet he made himself useful to society. It is reported that several times when he was recognized, he was a street sweeper or carrying water for people. Most likely, that was his way to earn a modest living and overcome his pride and love for his previous social status. Probably trying to follow the footsteps of the Imam Al-Ghazali, it was reported in recent years that a scholar from a renowned institution was accidentally found cleaning the Wadou (Ablution) area attached to a Mosque. Breaking one's pride and fondness of oneself is one of the first things new Sufi students do. They are typically discrete about it, because it defeats the purpose if they did it so that someone might notice them. According to Sufis, Pride is one of the factors that thicken 'the veil' that prevents one from 'seeing'. A person full of himself is not much different from a person full of anger or greed. It is preoccupation with negative feelings that Sufis try to break out from. And negative here means that such feelings are obstacles. Sufis have some time-tested prescriptions for overcoming this type of obstacles, and as I understand there are similar or other practices toward the same goal in other religions too.
In one of the incidents when Al-Ghazali was recognized, he was seen going to a Sufi Master wearing an ox's skin. It appears from the narration of the story that he did it specifically to pretend to be an ox. In Arabic culture, an ox is not considered a very intelligent animal. So there he was, a famous scholar who has proved himself to be knowledgeable and who has already written several books on Religion and Philosophy (before his serious exploration of Sufism), practically saying that when it comes to Sufism, he was as intelligent as an ox. I don't think it was lack of books on Sufism that could have led him to declare his lack of knowledge, rather it was probably what Sufis keep repeating that books can't get one too far in Sufism. A Sufi master said once that reading numerous books on Sufism for as long as Noah lived [950 years according to Quran] cannot make a reader a Sufi. Another Sufi master once warned against learning Sufism from someone calling himself a Sufi just because he read books on the subject. Analyzing concepts linguistically and logically is considered a very strong veil that prevents one from 'seeing'. A Sufi said that the moment he sees someone busy [or spends undue time] with grammar and eloquence [of Quran and Traditional sayings] he knows that that person has no knowledge of Sufism.
It was only when repeated Signs appeared, that Al-Ghazali felt he had to go back to normal social life. And he did that for a specific reason. When he came back he was ready to write what numerous people, including myself, found very helpful. This time, and unlike his previous writings, his books were not just based on information he had learned from teachers and books, but also on a deep personal experience. In my opinion, he is one of those writers on religion, who really knew what they were talking about.
Al-Ghazali's books are often described with praise even by those who have reservations about his thoughts. The way he structures his books, the logical progression of ideas, and his eloquence are all notable. However, most people, including those who defended his opinions, admitted that, while Al-Ghazali did not have any problems citing verses from the Quran, he depended on many unverified sayings of the Prophet. Now, to some critics that could be used as a good ground on which to reject his writings. Sufis however, regarded his books very highly, even though they knew about the unverified sayings. The reason appears to be that they were looking at Al-Ghazali's ideas, realizing that his attempts to find the appropriate sayings supporting the ideas, might not have been always successful. Several later scholars attempted to imitate Al-Ghazali's style in teaching religion, and they were very careful on how to select verified sayings of the Prophet, but none of those attempts ever reached the popularity of Al-Ghazali's books.
It is a rare exception when a Sufi master comes out of obscurity and intentionally talks to the public about Sufism, as I believe was the case of Al-Ghazali.
