Can I join the college just to do the book collecting quests for the orc? Or do they only become available after the main guild quests?

Yes you can if you so wish. There are some books that are only quest related, but I do not think those are the books you are talking about.

You may have to do the quest in Saarthal, but after that they should become available. (You should actually check before you go to Saarthal--they may be available beforehand.)

You don't have to go through the whole questline.

I'm pretty sure the only radiant quest is getting books for Urag, and he only gives like 20 septims for each book, not nearly enough for a college student to live on. Food prices have risen so high in Skyrim, ugh.

Well, you really don't get much from the quests that you couldn't get just from wandering around and finding all the locations yourself. On the other hand, it does give you some direction which can be good sometimes.

the reward is not the issue, just need an excuse to wander the land.