Another book I read for my online classes was Sacramental Theology: Means of Grace, Ways of Life by Kurt Stasiak, O.S.B.. This book reads quickly and is short in length (just over 150 pgs.). It is part of a series put out by Loyola Press/A Jesuit Ministry.
I noticed that my next class utilizes a book from this series, too. A great reference book for my religion shelf at school.