Yes, I make it a habit to go back every so often and hit the bookshelves in the Mages Guild. It also seems after you have reached a certain level in a zone that you will not receive any more points towards a skill line from the bookshelves.

Through the different experiences I've had, as well as the different experiences those I have played with have mentioned (granted, I don't make it a habit to ask everyone what their experiences are about book shelves, it just comes up), it seems to me that it's all completely random.

I seem to find myself getting ranks in skilllines I haven't even touched before, like Provisioning may hit level 2 or the like. Not that I'm arguing, but seems to hint more at it being purely random.

I have so many characters and try to play different ones with so many of my real life friends that I don't know which I have visited already and which I haven't - I just click on most any ones I see. I can't say by doing that, I have ever really seen a drought in gaining points.

They are random 100% i got 3 different skills from exactly same bookshelf in Daggerfall mages guild hall. But not after relogs, just few days later while visiting/doing quests. It happen several times since then. Can be related to server/daily quest restart, as 25 relogs(just to test it) gave me always same book.