This is hard to answer because bias is, of course, in the eye of the beholder.
Many people feel that Lewis is biased because of his very identity. The followers of Said's ideas (as laid out, for example, in Orientalism) would argue that no Westerner can truly speak about "the East" without showing bias. They say that this is particularly true if the Westerner holds political opinions that are different from those of many in the Middle East. So this school of thought would argue that Lewis is biased because of who he is.
There are also those who see bias in specific cases in the book. The minaret.org article, for example, argues that Lewis looks at Islam from something of an ethnocentric view and therefore thinks of Islam and Muslims as backwards.
Other people, however, believe that Lewis is fair and scholarly in his treatment of Islam. There is no way to objectively tell which view is correct