Robert Spencer is actually pretty accurate. Polemical, yes, but that doesn’t mean he can’t be accurate (I read Religion of Peace? and The Truth About Muhammad as a Muslim apologist, so I could issue rebuttals; I’ve read most of his other stuff since then). I have heard him speak and read some of his books, and while inflammatory and uncivil, I have not seen a glaring error in them. As far as popular speakers go, he’s the best around there is: far more balanced than someone like Tariq Ramadan (I call him the “Dinesh D’Souza of Islam”), son of Said Ramadan, close associate of Hasan al Banna who founded the Muslim Brotherhood or any of the other popular Muslim apologists.
Or the worst of them all, the useful fool or fellow-traveller Karen Armstrong - she does blatantly misrepresent the facts, and is like the inverse of Lorraine Boettner, dealing with Islam instead of Catholicism and blatantly misinforming and twisting and omitting - not just spinning - the facts, but in order to make it look good instead of bad (whereas Boettner just made stuff up about Catholicism to make it look bad, instead of good, and was guilty of the same kind of lying by omission).
Ignaz Goldziher wrote some good works on Islam while he was alive, and he was an Islamophile. Edward Said has nothing more useful to say than Ayn Rand. In general, those so-called Orientalists do have good knowledge of Islam, and the Orient: off of the top of my head, I think the best balance to Robert Spencer would be the moderately sympathetic Orientalist academic John Esposito.
When it comes to more rigorous and less polemical (although no more friendly) analysis of Islam, Patricia Crone, Michael Cook, Bat Ye’or, and John Wansbrough are the best academics working today, but they don’t do “Islam as Islam” - they focus on specific areas in the history of Islam, as they are academics: however, one can pick up any of their works and be assured of its quality.
Ibn Warraq does a good “Islam as Islam” in Why I Am Not a Muslim, and Efraim Karsh does a good historical overview in Islamic Imperialism: A History (actually nearly completely free of bias and somewhat sympathetic to Islam, despite the title).
And, there is no decent biography of Muhammad around. The earliest biography of him was from 200 years after his death, and is largely legendary. Reconstructing the Historical Muhammad is harder than the Historical Jesus. In order to get a balanced look, I would read two works, one from each side: The Truth About Muhammad by Spencer and Muhammad: His Life Based on the Earliest Sources by Martin Lings, the most straightforward and unbiased of all Islamic biographies/hagiographies of Muhammad (although it is still a hagiography, elements that are included and are seen by Muslims as points of pride, can often be seen by outside observers as points of evil, such as the cold-blooded murder of the 600 Jews after the Battle of the Trench): it is the most straightforward, “report what is recorded in the traditions” one there is. If you want to go straight to the source, all biographies of Muhammad are based on one source - the Sirat Rasul Allah of Ibn Ishaq (I am unaware if there is an English translation of this), sometimes with various admixtures of additional sunan (reported in the collections of ahadith) thrown in. Only the events that are or are not reported and the light they are cast in differ. The Sealed Nectar is another choice, but is a distant fourth to any of the above. The English translation of it also sucks, reading like a translated instruction manual.
In any case, stay away from tripe such as Muhammad: A Prophet for Our Times by the erstwhile “Catholic nun” Karen Armstrong, which is unabashedly apologetic (more so than even the annotation in the Yusuf Ali translation of the Koran), to the point the sweetness is tangible and sickening and the excuse-making is ridiculous.
**