Hi,
With respect to Hapkido, I'm not familiar with any such books. It's even difficult to find published articles that meet your requirements. The unfortunate truth is that much of the Hapkido history out there is just the kind biased, nationalistic puffery that you're trying to avoid.
We can say with confidence that Hapkido's origins date to the late 1940s and trace directly back to Choi Yong Sul. There is no serious doubt that Choi spent about thirty years in Japan before repatriating to Korea after WW2. There is no doubt that he was highly skilled in a style of Aikijujutsu that he had learned in Japan. There
isdoubt, however, about his account of having studied Daito-ryu Aikijujutsu under Sokaku Takeda for thirty years. No records of his training are known to exist.
The origins of the significant additions that Ji Han Jae and a couple of his contemporaries made to Choi's Jujutsu base are also debated. Some claim they are purely Korean. Others note Chinese influences. Fortunately, Ji is still alive, so it should be possible for serious scholars to contact him. Choi, of course, has been dead for a little over twenty years, was illiterate, and left behind little in the way of written records that have been made public.
From a technical perspective, a few of us Hapkido students have managed to spend time training in legitimate Daito-ryu dojos to try to see for ourselves whether Choi's purported connection to Daito-ryu might have any basis in fact. I can tell you that I, and at least one other guy (who has done a lot more research into these matters than I have) have concluded that it is quite possible that Choi learned Daito-ryu. There are some absolute technical parallels between what we believe Choi originally taught (which can still be seen in curricula like that of the Jungkikwan in Daegu) and Daito-ryu, including the use of aiki in the techniques. There are also clear areas of difference. All I'd be comfortable saying at this point is that the evidence that I've managed to see, in print and on the mat, is inconclusive.
For more on Hapkido's history, you might want to have a look at the history section of the Hapkido Forum website:
http://www.hapkidoforum.com/phpBB3/
There are some interesting, mostly impartial discussions there. Several of the contributors have done a good deal of research. I should warn you, though, that sometimes the discussions deteriorate into emotional arguments (nothing new there, LOL).
One final thought for now: there is a man in Geumsan whom Choi's family named Doju in 2002. His name is Kim Yun Sang, and his kwan is called the Yong Sul Kwan. He calls what he teaches Hapkiyusul rather than Hapkido.
There has been word for some time from one of his Australian students (who is a very reliable source) that one of Choi's daughters was about to publish her memoirs. These memoirs are said to contain a good amount of information that is said to confirm Choi's account of training under Takeda. As far as I know, the memoirs have not yet been published, although I haven't checked in a while. Also, they will probably not be published in English.
Sorry for rambling a bit, but hope this helps. The hard truth concerning Hapkido is that you're facing a formidable challenge.
Good luck to you anyway. :asian: