As if the DVDs and toys weren't enough, an official book is set to be released in Japan next month containing production materials, staff and cast interviews, and even glimpses at an abandoned sequel concept. You can preorder your copy here: http://www.1999.co.jp/eng/10200982 The maximum number allowed per household has dropped from 3 to 1 in the course of a few days, so I would assume they're getting a lot of preorders. Act quickly if you want a copy!

EDIT: That site has already stopped accepting preorders. If you still want it, Amazon Japan is still taking preorders.