The vitamin D council (a US organisation) has produced a booklet entitled "Vitamin D: Your guide to understanding vitamin D", which can be downloaded free from here:
vitamindcouncil.org/wp-cont...
Haven't yet read it myself, so can't comment on the content. Hopefully it will be helpful to those who are only just beginning to understand the significance of this long neglected health issue
I also don't know how long it will be available for free, or from the link above, so apologies in advance if you come across this post at some future date and find the link no longer works .