Time Displacement is mostly a one way form of time travel, and is always accidental. I have listed one book that has two way time travel, but it is the exception. Time displacement is when an individual, or group, is transported back in time and has to survive in the new enviroment. They might come from an advanced civilization, but the higher tech is generally not availiable without the infrastructure of the higher tech time period.
Islands In Time Series
This is actually two series set in the same world. The first three books follow what happens to the people on the island of Nantucket when it is sent back in time about 3500 years. The next books deal with collapse of civiliazation in our time that resulted from the alien energy used to send Nantucket back in time. The first three books would be classified as Time Displacement, while the follow on ones would be post-apocalypse with no gunpowder or similar.