Welcome and congratulations! I hear what you're saying about wanting to be content ready but I would strongly advocate for spending time engaging in what kind of history teacher you want to be. The specifics of what you'll be teaching will be up to your eventual school or district's curriculum, your state standards, and the specific grade level (9th is very different than 12th) so it's hard to get a running start on content given the high degree of variability.

However, in order to do right by proper historians, this is the perfect time to practice approaching history like a historian and develop muscle memory around how you'll present content to your students. One approach to consider is getting a textbook (many public libraries have textbooks from local schools - you can also buy old used ones fairly cheaply from places like ThriftBooks) from the content area or grade you think you'll end up teaching and start designing lessons. Get into the habit of building text sets using various sources and reading levels.

Get comfortable vetting sources. For example, Newsela has lots of current texts leveled up and down but they sometimes minimize complicated history for the sake of readability. I'd advocate spending time with some of the upper Lexile texts and familiarize yourself with what they have. Some other sources worth spending time with are projects like Teaching Tolerance's Teaching Hard History - the podcast is great - the Stanford Beyond the Bubble project and Zinn Education project.

Another good habit to get into is watching YouTube videos that proclaim to teach history and practice fact-checking or testing their claims. Regardless of where you end up, you'll likely be expected to teach students how to construct claims/counterclaims based on evidence and building up that muscle now so you can model for students what it looks like (and doesn't look like in practice) is good. Start building a book list of trusted sources related to various topics. (Our booklist can help with that) and use the work of fact-checking videos to build your own background knowledge.

Finally, it's also worth reading through this recent post from a parent and start to think about how you'll answer a parent's question about content their child brings home. A whole bunch of teaching history at the high school level is about the history teacher's identity and which aspects of history education they elect to lean in to. That is, do you want to be a teacher who maintains the status quo or one who pushes students to wrestle with a more complicated, more complex telling of history. Will you scaffold opportunities for students to challenge history and strengthen their civic identity? It is essential that students build accurate background knowledge in order to develop their critical thinking skills, but background knowledge alone isn't likely to satisfy high school students. They're going to want to do something with what your teaching so thinking about your assessment philosophy is also a good use of time.

All of that said, if you're looking for book recommendations, there are a few I would pass along. First, Clio in the Classroom is about teaching women's history but the content suggestions apply to all topics in history. Also, Why Learn History (When It’s Already on Your Phone) by Sam Wineburg is good. And I'm a big fan of Sarah Meta's Thinking About History if it hasn't crossed your radar.