How to say "red book"?
Both 红色的书 are 红书 correct, 红 alone has the meaning of 红色的(of the color red), for example:"那是一本红书"(That is a red book), in this example we use 红 to describe the color of a book, it's an adjective, its meaning is the same as 红色的. However, the character of 红 sometimes can be part of a noun which composed of two or more character, example: 红豆, 红茶, 红包, in this case 红 is a part of a noun, but it also implies that the thing is of red color.

If you want to describe a red thing, and you don't want to make grammar error, it is safer to use "红色的", because sometimes use 红 alone cannot complete a sentence, for example you can not say"那东西是红", that sentence isn't complete, you must say "那东西是红色的"(That thing is red).

The rule of above can be applied to other colors,for example:
橙chéng orange, 橙色的 of the color orange 
黃Huáng yellow, 黃色的 of the color yellow 
綠lǜ green, 綠色的 of the color green  
藍lán blue 藍色的of the color blue
紫zǐ purple, 紫色的of the color purple