Those wishing to do serious desktop publishing, including books, in scripts like Arabic, Devanagari, or Tamil in OS X are handicapped by the fact that standard applications like Word, InDesign, and Quark cannot yet render OS X Unicode fonts for these languages correctly. Alternatives are the limited TextEdit or Pages (which is buggy for Arabic), or using Windows fonts with OpenOffice. There does exist a special ME version of InDesign for Arabic/Hebrew.
Recently I came across a DTP program called iCalamus that appears to combine a wide range of DTP capabilities with proper rendering of complex scripts. Users with this kind of requirement may want to download the free trial here and give it a try. Unfortunately iCalamus does not yet support direct input and editing of Indic -- you have to copy/paste your text from TextEdit or another app where this can be done. A somewhat similar app which does support direct input is Create.