Books (sometimes called monographs) are secondary sources that provide analysis of primary sources and/or other secondary works. Historians, and other scholars who study the past, use both books and journals to conduct research and to publish their own findings.
Types of Books
Scholarly monographs examine a topic both broadly and deeply and make an extended argument or arguments. They are distinct from books which are collections of essays. Collections may be a compilation of essays or shorter pieces on a single topic, or the writings of one scholar on many topics. A collection of essays in honor of a scholar is known as a Festschrift (celebration writing).
Most scholarly articles either publish original research or findings, or, summarize or review other research on the topic.
Scholarly
There are many ways to tell if an article is from a scholarly journal (i.e. has undergone peer review), but you can always look it up in Ulrich's Periodicals Directory if you are not sure.
Peer Review
The most important aspect of a scholarly journal is peer review. Here's a link to a tutorial about peer review (3 min).