IDOL FILE magazine is about to release Vol. 2 Tokyo. The date is set for February 17, 2017. It appears they cover all different types of idols. I have not actually seen a previous copy, but I assume it is pretty standard fare. Probably a short article and a photo spread for each group. you will be able to pick up a copy from Amazon.jp or Tower Records, to name a couple.
Here is a short list of some alt idol groups appearing.