1.用is of great help没错,即：
The dictionary is of great help to my translation
= The dictionary is very helpful to my translation
即：of + 抽象名词 = 该抽象名词对应的形容词形式
2.如果用a great help也可以.
此时的help视为可数名词,意思是“有帮助的事物”.
很多不可数名词可以转换为可数名词使用,且此时其前经常有形容词出现.
再如：He is a great danger to us all.
3.第二空就不多解释了,be of no use = be not useful