And then there are fix-ups. Hard to forget when you just read the first volume (perhaps they were grouped in a omnibus!), but much harder when to you, the stuff was five pages earlier. Perhaps they were published years apart, but by juxtaposing them in a single volume, you ensure that the reader notices that you are repeating yourself.
Exposition is an underrated albeit tricky form of infodumping -- but it's particularly tricky for this kind of infodumping, as its great weakness is flatfootedness, and flatfooted relations of stuff you already know are particularly dull. Slithering it into the background or motivating the characters to say it are more important than usual.
Then, there are the in-jokes. The reference that is only clear to those who have read before. But if it is not invisible to anyone else, if there is no explanation of why we get this discussion of this cameo -- they can be very annoying. In fact, if they would be noticeable to the new-to-the-series reader, it doesn't matter whether I read the other work: they annoy me. In-jokes should be either fleeting or integral to the work in which they appear. Which doesn't mean that they will necessarily be explained. If only the readers of your first book know why the shopkeeper has eyes that don't match, still, the shopkeeper can be an integral part of the second book without mentioning that they have met before.
Which is, in fact, a general rule. Be wary of explaining too much of What Has Gone Before, when What is Happening NOW is the crisis.