After you've read a high-level overview of your topic in an encyclopedia, you can use the keywords you found to locate books and ebooks that take a more in-depth, thorough look at that topic. Whereas encyclopedias must cover a large variety of subjects, scholarly books tend to have a very narrow focus, allowing the authors to provide a more nuanced and precise take on a subject. Especially in fields that do not change rapidly, they may be where experts publish their authoritative research, comprised of many years of study and writing.
Scholarly books are not written like popular novels, where to understand the fifth chapter, you need to have read the first through fourth chapters. When reading a scholarly book, you'll notice that some sections may be more relevant to your studies than others. When working with this kind of text, it's OK to jump around or narrow your focus to one or two chapters that specifically address your research.
In addition, books can be great places to seek out bibliographies and reference lists! Much like in encyclopedias, the experts writing the book are pulling their information from a variety of sources, which can be very valuable when becoming an expert, yourself!