Why has Crooks been able to accumulate more personal items than the other ranch hands?
Crooks also represents a different sort of common laborer than do the other male characters. He does not seem to be a migrant worker (someone who moves from job to job). Judging from Candy's stories about Crooks and some of Crooks' comments about his own disillusionment, he has been at the Tyler Ranch for quite a while. When one does not have to move from job to job, "home" to "home," carrying his "bindle," then it is easier to keep things that he might not normally hang on to.
Similarly, because Crooks does not have any real relationships with the other men, he does not have an opportunity to trade his books or other possessions with them. His isolation makes it necessary for him to form a bond of sorts with things rather than people; so during a time period when many Americans had very little, Crooks clings desperately to his possessions because he has nothing/no one else to cling to.
I have a few thoughts on this, mostly having to do with race.
First of all, Crooks is probably more likely to save his money than the other hands are. This is because he is probably not welcome in the bars and whorehouses that the other men go to. If those places won't let him in because he's black, then he's forced to save his money and can buy other stuff with it.
Second, since he has his own room, he has a place to put stuff. The only reason I can see for him to have his own room is because he's black and the others won't want to share a room with him.
Being separated from the other guys who share a bunkhouse affords Crooks the need to have more things. The other guys refuse to live with him because he stinks, he admits this to Lennie. Thus, they can share a deck of cards and books and magazines... Crooks doesn't have the privilege of other people's items.
The fact that he's been alienated making him accumulate more things should be a lesson to the other ranch hands. He is profiting materially from their discrimination.