You can have as many as you like but you can only ever summon 3 items from tomes per day (edit - you can only use tome skills 3 times per day, forgot that one of the tome skills summons multiple items...I'd say sugar shummoning is the worst of the tome skills
).
So I got the sugar shummoning one thinking 'well its better than not have anything'. I've since decided that unless you're in hardcore and want to make use of the weapons and them breaking into shards you can eat for health and mana, sugar shummoning isn't actually that good.
However this means that the relative benefits of me buying another tome have now gone down drastically because in order to get decent use out of the new tome I'd have to use my old tome less. Obviously its still slightly better as you get more options but generally I'm just
I kinda regret not getting Sorcerers of the Shore Grimoire when that was available. And unless I think its rubbish I'll almost certainly get the next libram that comes out.
I still might be tempted but already having a tome does make the item less awesome. As I said, if you don't have one I'd definately think this is a good IotM if only because unbearable light would speed ascending up quite a bit
plus other fun buffs/food/drink