They are different books. They have the same general purpose, but they take a different approach.
The Baltimore Catechism was published in the 1800’s- that was before two world wars, the rise and fall of the Soviet Union, the cultural turmoil of the 1920’s and the 1960’s, the Second Vatican Council, and the 1983 Code of Canon Law (which changed some of the things- like rules of fasting before Mass- that are mentioned in the Baltimore Catechism). While doctrines have not changed, the approach to many things is different (relations with the Jews, for example). It’s good for Catholics to know, because it has some very sound truths in it, but it’s probably something you should read after you know more about the Faith.
The Catechism of the Catholic Church was published in the 1990’s. It isn’t as straightforward as the Baltimore Catechism- it goes into more detail in explaining why the Church teaches what it teaches. It references the Second Vatican Council, which had not happened at the time the Baltimore Catechism was published, whose documents- whether people like them or not- have done a lot to shape the Church as it stands today.
Since you are looking for more information on the Catholic Church, I should say the Catechisms alone might not be enough. Books like “The Journey Home”, “Surprised by the Truth”, and “Catholicism and Fundamentalism” helped me understand things that the Catechism just lost me on.