The best Islam books stretch from a seventh-century Arabic scripture to advice on getting through an ordinary week, and that gap decides which one to open. A scholar's overview and a book of daily encouragement sit under the same heading, and picking the wrong one wastes the purchase.
Someone with no background at all should begin with Islam: A Very Short Introduction, which handles the divisions, the law and the modern politics in about two hundred pages. A Muslim buyer who wants the faith addressed rather than explained is better served by Reflecting on the Names of Allah, taken one name at a time.
Books bought under this heading do very different jobs. The source texts are consulted rather than read front to back, the histories and introductions account for where the religion came from and how far it travelled, and a large contemporary shelf is aimed squarely at Muslims looking for help with the practice itself.