There's the usual interview with the Great Cow Spirit, summaries, gaiden manga (this one featuring Izumi), and.....a section on the military people. Such as how Havoc, Farman, Breda and Fury ended up in the military. And the identity of the elusive Chess General, which some of us called Cameron.
Source : http://momodori.seesaa.net/article/3191587.html
If there's anything wrong with Havoc and co, I apologize. I haven't seen the text itself and am a bit leery of translating what I haven't seen personally, but that's the gist of what the blogger wrote. He's about the only one who goes into good detail that I can find on short notice. But the entire JFandom agrees on the Chess General bit, so it's probably correct.
In the case of Jean Havoc : He was something of a problem child. He attended Officer Cadet School (the British called it that, IIRC, and Arakawa says she follows the British system, so I opt for the British names unless otherwise confirmed), but because he's the type who prefers brawns rather than brains, he graduated with near-flunking scores. But since cadet graduates are automatically relatively elite, he doesn't have much of a problem. (I'm not sure if this is just the blogger's view on it) As for the Ishvar Civil War, he did not participate. 1914 cadet graduates are automatically Second Lieutenants upon commission, so this might mean that Havoc had always stayed where he was/never got a promotion, if Arakawa would follow history that much. Maybe he started off being put in Roy's command, I wonder?
In the case of Hymans Breda : The blog didn't say much about him, unfortunately, save for the fact that he appears to have done the same thing as Havoc in the same year, and they've been friends. Classmates at cadet school? Breda would've passed with flying colors, though. The man's smart. Also, the two obviously had good relations, as he was willing to work away his paid vacation for Havoc's sake. (When is this? Gaargh)
In the case of Cain Fury : Fury enlisted as a.....general recruit applicant....a volunteer, I guess. No more was said canonically, though the blogger did suggest he might've joined to use his field of specialty, as it was revealed in Guidebook #1 that commercial wireless communications in Amestris isn't exactly a booming business.
In the case of Vato Farman : After graduating from Officer Cadet School (ah, I guess Arakawa's not so strict about the ranks, then. Warrant Officers are kinda like an inbetweener class in the west, neither officer nor NCO. You wouldn't graduate into one, normally, IIR) he was transferring into Eastern Command, whereas he was put in Roy's unit. Before moving, he has wondered a great deal about what kind of CO he would be serving under. (The kind who lost the entire unit's paper clips supply, of course). Since he's a very intelligent guy....I always will be interested in seeing a genetic mix between him and Sheska, one of these days....he breezed through the written examination, but due to his frailty, firearms and physical qualifications were difficult as he couldn't quite handle the recoil. (I'm not sure whether this is just a blogger comment, though)
In the case of the Chess General : Riza Hawkeye's grandfather, on the mother's side of the family. Nuff said. I'll leave you to draw your own conclusions, as I'm still drawing mine at the moment. (Although the line about Roy and 'my granddaughter' in Vol.6 is......)
The perfect guidebook itself can be found in Amazon.co.jp. I want, dammit!
( amikitty, I posted it. Is everything alright with the world now? XD; )
Also : According to the FMA Complete Materials : Story Side book, Riza is four years younger than Roy, who is the same age as Hughes, for anyone who's wondering. His onscreen deeds in Ishvar was done when he was....22, I think. It's either 22 or 23...the exact year eludes me for the moment. Yes, what he did in episode 13 and 15 was in the same year. The end of Episode 25 was the last event to occur in 1914, after Ed reaches Rush Valley, the book lists the year as 1915. For those wondering. It does say a few things about the climate of Amestris, I guess.