As above, conditions outside their ideal range will make the fish loose their full colouration, but with careful aclimation, can still be maintained. If for example you placed Malawi cichlids needing a pH of 8 into a tank with a pH of 6.5, they would realy struggle, becomming suseptible to disease and with the extream difference in this example, they can die. A difference of +/- .5 of a point won't make too much difference though
Discus reading material.
Anything that isn't a book realy

As with most fishkeeping books, the discus litriture available in paper form is usualy arround 30 years out of date, and 30 years ago duscus were theought difficult to keep even for the experienced. Now we actualy know more about them, even beginners have been known to be successful keepers. Read all the on-line sites and arround the forum and you will get an information overload that is up-to-date

Once you get to keeping them though, scrap the research and replicate an existing set-up.
These are "advanced" fish and don't read out research

They often don't like common methods of keeping them

If the copy of an exising set-up isn't working, then tweak it according to the gut instinct the research will have given you. However, if it ain't broke, don't fix it

Many on here recon that they need a low pH and hardness, with 2 30% weekly waterchanges minimum. Mine flake over with more than 1 20% change a week, and go into super-strop mode if the pH falls below 7.3
All the best
Rabbut