The FEniCSx documentation is available at https://docs.fenicsproject.org/.
The FEniCSx development roadmap summarises planned features.
The FEniCSx tutorials provide a range of tutorial programs as Jupyter notebooks.
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 The book was published in 2012, and was based on the legacy FEniCS library, so the code examples in the book are out of date.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 the many of the concepts in FEniCSx.
The book is available as a free ebook, or can be bought from Springer or many other bookshops.
Legacy FEniCS documentation
Documentation for the legacy version of FEniCS is available at https://fenicsproject.org/olddocs/.