Every Catholic apologist needs to have this book on their shelf for reference. It covers the breadth of Catholic dogma well. Explanations are concise but what they lack in depth is made up with abundant references. Most questions can be answered with the content contained in the book. When more information is desired, the references are good starting points for exploring the more subtle aspects of a particular dogma.
Ott's compliation of Catholic systematic theology first appeared in German in the early 1950s. Translated wonderfully into English since then, it remains the best single volume presentation of Catholic theological data (as opposed to, say, catechetics) available anywhere. Indispensable, extremely easy to use. Everybody who's anybody should have a copy on the shelf somewhere.
Ludwig Ott's book is the type of book all Catholics need to have within their homes besides the Bible and the Catechism. It explains, goes into history, gives quotes from the Fathers an Popes of the Church, and other useful information.