You can download the current version (0.6-9) of the User Manual as a PDF here.
The manual provides information for those wishing to use NIMBLE to work with their own models as well as algorithm developers wishing to write algorithms using NIMBLE.
The following paper describes the motivation and design of NIMBLE:
de Valpine, P., D. Turek, C.J. Paciorek, C. Anderson-Bergman, D. Temple Lang, and R. Bodik. 2017. Programming with models: writing statistical algorithms for general model structures with NIMBLE. Journal of Computational and Graphical Statistics. https://doi.org/10.1080/10618600.2016.1172487.