It is considered that diagrams make chess books easier to read, but there are obvious space limitations to how many of them can be included - mostly in printed books. An ebook has less of a concern for maximum number of pages, but is it still possible to make readability worse by having diagrams sprinkled too frequently in the notation?
Let's start by saying that that's a subjective question. What is "too many" for one person may be just right for another.
That being said, let's take a ridiculous case: A diagram for every move, whether main line or variation. The diagrams would get in the way of the exposition. It's safe to say that very few chess players would benefit from such a book.
The "most friendly" chess books I have read include diagrams every 6-10 moves of the main line only, whether with or without exposition. There is a reason behind this. Buffering and Human Memory describes this phenomenon, with a full paper called The Magical Number 7. Briefly, humans can remember 7+/-2 discrete items without having to switch knowledge in and out of "storage".
This will also arrange matters such that a reader can follow the play without a board, but make it desirable nonetheless that a board be used. In addition, if the reader makes an error in replaying moves, 6-10 move intervals make reasonable "checkpoints" to return to.
The problem is not so much with the diagrams, but the amount of real content and how it is presented and organized (like proper indexes, a lot of books fail this criteria even today). Chess books of this day tend to have more diagrams but less content (as compared to the time of Batsford), so they can appear to be more readable but with little benefit, as they don't even have enough content to fill 200 pages most of the time. (not counting monographs such as ECO and Informators).
Except for beginner books and speciality books like puzzles, number of diagrams doesn't factor much in terms of readability. To properly read (or follow) a typical book, there's no real secret (or should I say shortcut), it's just a matter of having either the board vision to play out variations in your head, or diligence with the computer/a real board.