Excellent notes by Strickland
There is some good material in the green book
See Kedlaya notes also, on one of his course pages.
Yoshida: a proof of local CFT
Lazard: Bourbaki exp 76
Bondarko has a number of papers on formal groups, for example a paper completely classifying formal groups over rings of integers in local fields.
LNM0074 Frohlich. Chapters on Lie theory, Commutative FGs of dim one, Comm FGs of dim one over a DVR.
Loday on the operad of associative algebras with derivations, formal group laws, and more http://arxiv.org/abs/0906.4730
http://ncatlab.org/nlab/show/Lazard+ring
nLab page on Formal group