dunno about skinny but before donate blood, they will timbang you and check your blood pressure

too high/low bp - kenot donate
too skinny/fat - *maybe* kenot donate as well

the red small book if for record purposes. you donate multiple times got this free benefits like 1st class ward treatment etc2. just to encourage people to donate

oh you have to complete a set of questionnaire also before they they your blood... something like gay, blood transfusion, free sex, drugs etc to filter against high risk disease like AIDS / hepatitis or like that

if you donate a number of times you can get free medical treatment.

they will do some sort of blood coagulation test,
to see how well your blood clot upon pricking/injuries

if u blood clot fast enuf, its ok to donate even if u're skinny.
otherwise u bleed like hell..