drivers/iio/, contains the IIO core plus and directories for each sensor type (e.g. accel, magnetometer, etc.)
include/linux/iio/, contains the header files, nice to read for the internal kernel interfaces.
include/uapi/linux/iio/, contains files to be used by user space applications.
tools/iio/, contains tools for rapidly testing buffers, events and device creation.
drivers/staging/iio/, contains code for some drivers or experimental features that are not yet mature enough to be moved out.
Besides the code, there are some good online documentation sources: