HTM is Old Calendarist.They were founded independently (the comemorated the JP at the time) in the early 1960s, and by mid-decade became associated with ROCOR. They split with ROCOR after a scandal in the mid-1980s, and supposedly have ties with Esphigmenou on Mount Athos (though that is a claim not exclusive to them amongst Old Calendarists).
The prayerbook contains the major services of the church day and some akathists. It's fine for use, but as others have noted it's hard to spit out the language, which is faux-antiquated.
In the mid-1980s, HOCNA did split from ROCOR when ROCOR tried to investigate allegations of sexual improprieties involving one former Elder Panteleimon. In 2012, this same elder recently resigned from the priesthood after he had admitted guilt in front of his brotherhood. At the same time, when HOCNA openly embraced the heresies of Awake Sleepers (Universalism) and Name Worshipping, about half of the brotherhood then split from HOCNA and joined HOTCA/GOC, which has ties with Esphignmenou on Mt. Athos. Recently HOCNA has gone into communion with the so-called GOC of AB Makarios of Greece. This Greek Synod also accepts the Name Worshipping Heresy. Therefore, HOCNA and AB Makarios' Synod in Greece have both become heretical and are no longer Orthodox Christians. With this in mind, it is spiritually dangerous to purchase any new prayerbooks from HOCNA.
Besides, with few to staff the HTM monastery businesses (icon, incense, and book press), it may be difficult to obtain their products.RE: HTM prayer book
The main difficulty I have with all of HOCNA publications is the fact that their prayerbooks and psalters have continued to undergo minor changes in wording with each publication. Thus, my husband and I have different editions with different paginations so that when we pray together, I simply cannot tell him the page number, but I need to take his book, find the page for him, and then hand it back to him. If I read a prayer in my version of the HTM prayerbook, the change is often enough for him to become confused so that he repeats the prayer. Thus, HOCNA books are a terrible distraction.