Since XML has recently moved from document-center environment to
data-centric environment DTD's are of less importance when compared to Schema.The strength and weaknesses of DTD's and Schema is very well explained in the book"XML Schemas" by Chelsea Valentine, Lucinda Dykes and Ed Tittel.The book deals in a precise manner on DTD and Schema structures and functions with examples.
It deals with document -Focused versus Data-Focused and also has a very good argument on why Schema are better than DTD's.