II think Richard McElreath’s “Statistical Rethinking” is a fantastic framework for them conceptually. The chapter that introduces mixed models is actually the free sample on his website: https://xcelab.net/rm/statistical-rethinking/
The brms package is very good for fitting, it has a nice syntax for mixed effects and gives you full Bayesian output. I also tend to use mgcv::gam a lot as it handles simple mixed models very well and incorporates a lot of other useful features when I don’t need full-blown MCMC estimation. Plug: I’m co-author on a recent paper on using mixed models and GAMs: https://peerj.com/preprints/27320/
This is a fairly broad question but Ben Bolker posts a lot of great material on rpubs: https://rpubs.com/bbolker
See also his paper on the topic that provides a great review including a R packages and etc: Bolker et al 2009 “Generalized linear mixed models: a practical guide for ecology and evolution” Trends in Ecology and Evolution (pdf)
Then there is his text Bolker “Ecological Models and Data in R”
See also
Kruschke “Doing Bayesian Data Analysis: A tutorial with R, JAGS, and Stan”
Gelman and Hill “Data Analysis Using Regression and Multilevel/Hierarchical Models”