All three of these books have much more in common with aikido than with any authentic Daito-ryu Aikijujutsu.
The Hidden Roots of Aikido by Shiro Omiya does have a brief overview of some historical information about Daito-ryu. I don't recall off the top of my head how accurate it is, if I remember correctly the info is fairly common and well known.
I have two problems with this book - first, Omiya's Daito-ryu credentials are not clearly disclosed in the book. It says his teacher was Kozui Tsuruyama who was a 2nd generation disciple of Sokaku Takeda. This statement is misleading - did Tsuruyama actually train with Sokaku or with one of Sokaku's students?
According to Seigo Okamoto (headmaster of the Roppokai branch of Daito-ryu) Tsuruyama did participate (he demonstrated jo techniques) in the demonstration commemorating the 30th anniversary of Sokaku Takeda's death hosted by Tokimune Takeda, and after that he came to Kitami (to the Kodokai) and practiced outside there with Okamoto sensei (who was at that time the senior instructor in the Kodokai under Horikawa sensei). They took photos of techniques for a book that Tsuruyama wrote. The book was published with the title "Aikido". Tsuruyama then agreed to publish another book using Kodo Horikawa's name. Okamoto sensei sent him about 100 photos as reference material for that book, but he never contacted them again!
Tsuruyama says in his book that he learned his techniques directly from Kodo Horikawa sensei, but according to a letter from Horikawa sensei's wife, Tsuruyama only took one lesson (about one hour) from Horikawa sensei personally during one of his visits to Tokyo. Tsuruyama also claimed at one point that he had received a license or scroll from Horikawa sensei, but it's doubtful since he originally referred to it as a Daito-ryu menkyo kaiden, but then later referred to it as a Nihonden aikijujutsu menkyo kaiden.
Secondly, regardless of Tsuruyama's credentials, Omiya's book claims to reveal for the first time in English a detailed visual introduction to "secret" techniques of the Daito-ryu Tradition. And it claims to be an indispensible tool for students of Daito-ryu.
As a practitioner of Daito-ryu, I would argue that the techniques in the book are more closely related to aikido than to Daito-ryu Aikijujutsu, and certainly do not constitute it's "secrets". Having said that, the book may be of interest to aikido practitioners for a slightly different perspective on standard techniques and perhaps some less familiar variations.
"Unlocking the Secrets of Aikijujutsu" by H.E. Davey has absolutely nothing to do with Daito-ryu. Davey's background appears to be in Shin Shin Toitsu Aikido (Tohei style), and some Nihon jujutsu. I've not seen any convincing evidence that a Saigo-ryu ever existed that was anyhow related to Sokaku Takeda, Saigo Tanomo, Daito-ryu or the Aizu clan.
Technically there's no "aiki" evident in any of the photo's in Davey's book. Nor are there any real "secrets" to be found there either. Davey's skills as a martial artist may or may not be good (I've never met him), but his book is misleading and the history of (Saigo-ryu) is dubious at best.
Samurai Aikijutsu by Toshishiro Obata was one of the earliest books in English that mentioned very much about Daito-ryu. Obata's credentials are legit in that he studied with Yoshinkan Aikido founder Gozo Shioda. His Aikido is heavily influenced by this prewar student of Ueshiba. The Yoshinkan style of aikido maintains the more practical "martial" approach of Aikido's formative years and the Aikibudo period.
It was Obata's contention that since Shioda learned from Ueshiba during his prime years - not long after Ueshiba had been affiliated with Daito-ryu that the techniques preserved in the Yoshinkan are more akin to Daito-ryu Aikijujutsu (or Aikijutsu as the book's title called it), and thus ancient samurai techniques used for real combat.
According to a student of Obata sensei, the book's title was chosen by the publishers and that Obata had little control over what finally appeared in the book. Since he had only recently moved to the States at the time it seems the publishers took advantage of his generosity and perhaps his limited English ability. Obata sensei readily admits that recent historical research has shown a number of errors in his history, but that he wrote basically what he had been able to find out at that time.
I personally think the book is not a bad choice for aikidoka particularly for beginners, because it advocates a serious and practical approach. However it doesn't really give a clear or accurate picture of Daito-ryu.
I also think that the term "Aikijutsu" is a bit of a misnomer, in that it isn't used by any of the Daito-ryu practitioners that I know. In America, it's use stems more I believe from aikidoka seeking to remove the emphasis from the softer "do" or philosophical path, and replace it with a more practical hard-style "jutsu" approach.
In Japan it's sometimes used as an abreviation of 'aiki no jutsu' or "techniques of aiki". These are among Daito-ryu's most advanced and effective techniques, however, contrary to popular opinion they are even more soft and relaxed than most soft style aikido techniques. People who are looking for a harder style of aikido should look towards jujutsu, Daito-ryu's aiki techniques are something else altogether. Because other fradulent schools and instructors have adopted the term "aikijutsu" to describe their versions of combat aikido+judo+karate garbage, I prefer to use the proper terms "aikijujutsu" and "aiki no jutsu".
Pretty much all of the aikido curriculum is derived from Daito-ryu JUJUTSU rather than from Daito-ryu's Aikijujutsu or Aiki no Jutsu. Most people fail to understand the differences and distinctions between jujutsu and aiki. In Daito-ryu they're quite different.
Anyone interested in Daito-ryu Aikijujutsu should purchase Stanley Pranin's book "Daito-ryu Aikijujutsu: Conversations with Daito-ryu Masters" Published by Aiki News. It is the most authorative well researched book available in English on Daito-ryu. Another good book is available from the same publisher by Katsuyuki Kondo senior instructor of the mainline branch of Daito-ryu. It features the Ikkajo series of techniques the first basic techniques of Daito-ryu Jujutsu. The title slips my mind at the moment. They're both available from:
Likewise, if anyone is interested, Seigo Okamoto headmaster of the Roppokai branch of Daito-ryu has some excellent seminar video tapes available in English. These tapes show real authentic Daito-ryu Aikijujutsu and secret "aiki no jutsu" techniques revealed for the first time and explained in great detail. Anyone interested can email me.
[This message has been edited by Brently Keen (edited 06-29-2001).]