I have never heard of Ovason before, but Pat Z's stuff is great. I haven't read inner order teachings yet, should have bought it when it first came out, was only twenty bucks then.
I do have the Z-5 books and his others. All great, but unless your interested in serious Golden Dawn material it will not interest you. As you can tell from the title it is solely dedicated to the Z formula. But if that is your bag you cannot do better. Has the Whare Ra versions of the neophyte and zelator rituals which are slightly different from the stella matutina ones. Also a lot of analysis of the rituals and covers the officers and invisible stations as well as goes into depth about the tools involved. Not their construction but their actual use and symbolism which your just not going to find in other texts.
From your link I think the Z-5 book 1 book is over priced. You should be able to find it cheaper on ebay in the $15-$20 range. Plus a pdf copy of both books are in the SM Library.
Once you get them and find that you like them, pick up a copy of his Equinox book and the Qabalah one as well. Both great.
EDIT: Forgot to mention, book 1 just covers the neophyte ritual and book 2 just covers the zelator. The rest of the grades were covered in a complete copy, commenly called the white brick. Impossible and expensive to find. There was talk of reprinting it but so far hasn't happened. If you write to him I believe you can buy it on CD.
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