Thanks for the feedback, and your topics of research might very well make books from the 1800s irrelevant to you. However, for other research topics, some books from the 1800s (and earlier!) may still be relevant and useful for research. They could be primary sources, for example, representing a particular time period. Or they might be landmark books (in literary studies, history, or classical studies, to use three frequent examples) that are as important today as when they were written.