Robert Spencer is a renowned scholar on the subject and not a liar as commented by some reviewers. Why are muslims frowning about whenever they find questions hard to answer? Even imams are stuttering for answers when debating with the author. Instead of accusing the author and other authors on the subject, why don't muslims check whether the facts stated (with clear reference to the Koran and hadiths ; the reference numbers may change because different versions of Koran has different numbers for verses, but the verses are there)? If you are bold, show your bravery in knowing the ultimate truth.
I recommend readers to buy and read "The Story of Mohammed : Islam unveiled" by Harry Richardson as a precursor to this book.. This one is more simple and gives a better starting block before you read "The truth about Muhammad"