Good question, especially because al-Biruni is one of the enigmatic figures of Islamic scholasticism, in so far as dating and verifying his works has proved a difficult task. Dimitri Gutas, of Yale, has worked extensively on this issue and he concludes that several famous essays, debates, and commentaries are only written under his name by a pseudonymous author. However, the issue becomes complicated because several respected 3rd and 4th generation students of al-Biruni took the 'fabricated' works as valid and studied them as though they were al-Biruni's.

But I don't think that's what you are asking. I think you are wondering how veracious his statements are about India. I will say this. I briefly studied his Kitab al-Athar (Books of Past Centuries) with Dr. Asad Ahmed, and have found his research methods, criterion for hisorical validity, and his general thoughts on history to be well beyond any of his contemporaries. Moreover, he is world class historian and should be used as a reliable source.

His was a Turkoman, and was Persian trained, so of course when he travelled to India after his studies he also brought his Persian/Hellenist background. But I would argue that al-Biruni can be relied upon for his reports about India because he did not travel to India to study them as objects or subjects of an empire like Orientalists, rather he was researching Hinduism and its connection to Neo-platonism, and the cosmogony of the Hindu's. In that regard, al-Biruni is a trusted and sincere historian to me and, as I have seen, the greater academic community.