Meaning of "need but read"?

I saw the sentence something like "One need but read the depressing accounts of how people lived in London and other large British cities early in the 20th century to be grateful that the good old days are past."

I can partially understand what this sentence means but can not see whether it is correct in terms of grammar.

What does "need but read" means?

It means "One only needs to read". However 'One' used like this is a third person singular pronoun like 'he', 'she' or 'it' so the sentence should read "One needs but read..." rather than "One need but read...".

The Merriam Webster dictionary has a number of definitions for but, the most relevant of which in this context is 2c

with the exception of — used before a word often taken to be the subject of a clause

Which has the example

none but the brave deserves the fair

This example can be reworded as

Only a brave men deserves a beautiful women

Again 'but' is used in the sense of 'only'