Comparative adjectives cannot have -er ending [closed]

Questions on the use of lesser have appeared on here before, but I was reading a book on grammar which states the following (I omitted parts to keep it brief, but I retained what I think are the important parts):

"Adjectives which gave a comparative or superlative signification do not admit the addition of ... the terminations, er, est ... the following examples break this rule:"

It then lists examples. Here is the one I am confused by: "From these various causes, which in greater or lesser degree, affected every individual in the colony, the indignation of the people became general."

The book sets "lesser" in italics, suggesting this is in error, yet my search on-line doesn't seem to yield any issue with the use of lesser. I found questions discussing the use of less versus the use of lesser, with it being argued that lesser is used for 'quality'. Here, I assume that 'less' is more appropriate.

My questions are as follows:

Should 'less' replace 'lesser' in the phrase "greater or lesser degree"?

Is the book claiming that 'lesser' is not a word?

Based on your excerpt above, I believe what this book is trying to say is that the word "lesser" is an EXCEPTION to the normal rule that you cannot add -er or -est to an adjective that is already a comparative or superlative. That is, "lesser" IS a perfectly good word, even though it breaks the general rule.

The point, in case it's not clear, is that in general to make an adjective a comparative we add -er, and to make it a superlative we add -est. For example, "John is tall. John is taller than Bob. John is the tallest man in the room." "Tall" is a simple adjective that describes something about John. Adding -er makes it a comparative. Now we are saying that John's tallness is more than Bob's tallness. Adding -est make is a superlative. We are saying that John has the most tallness of anyone in the given group.

But if an adjective already indicates a comparison without adding the -er, we don't add -er to it. For example, "worse" is already a comparative. We can say, "John is worse at math than Bob." So we don't add -er to it -- we don't say "John is worser at math ..." Similarly "worst" is a superlative, so we don't add -est to it. We don't say, "John is the worstest", just "John is the worst".

But in the case of "less", "lesser" is also a perfectly good word, thus breaking the general rule.