If you have questions about how to use FEniCS, you can find links to how to do this on the community page.
Full documentation of the components of FEniCSx is available at https://docs.fenicsproject.org/.
The FEniCSx tutorial provides a range of tutorial programs as Jupyter notebooks.
FEniCSx vs legacy FEniCS
FEniCSx is the latest iteration of FEniCS, and boasts a number of major improvements over the legacy library. We recommend that new users use the latest release of FEniCSx. The FEniCSx development roadmap summarises planned features.
Documentation for the legacy version of FEniCS is available at https://fenicsproject.org/olddocs/.
The FEniCS Book
The book Automated Solution of Differential Equations by the Finite Element Method explains the theoretical background and design of FEniCS. It describes the FEniCS software components in detail and showcases a number of applications of FEniCS to problems in fluid mechanics, solid mechanics, electromagnetics, and geophysics. The book was published in 2012, and was based on the legacy FEniCS library, so the code examples in the book are out of date. However, the book still gives a good description of many of the concepts in FEniCSx.