We all surmise what grammar books say, but I wonder how many of us ever look at an English grammar book that is the same size as some of the Greek and Latin grammar books we use. I know I never do. They are unwieldy, badly organised, and it can take an aeon to find what you want.
Something that is quite interesting is far, farther, further, furthest, farthest.
I once saw Jonathan Ross shouted down by a blue-stocking on telly. He had probably said "farther", and she shouted, "FAR, FURTHER, FARTHEST!" at him. The OED devotes a fair bit of columnage to this, and it doesn't tell you what the conclusion is - you have to work it out.
The trouble with the blue-stocking is, you can be drilled at school in a manner that convinces you (and one of the things you learn at a top school is that your view is superior to anyone else's) but that doesn't make it right.
Chambers has no preference between farther and further and farthest and furthest, and the OED is inconclusive, as I say. Onions seems to prefer farthest, so that's a start, but he doesn't say much about farther vs further.
I only decided that I grudgingly preferred "far, further, farthest" after reading the OED in conjunction with a book on Anglo-Saxon.
There is an Anglo-Saxon comparative "further" (not the original spelling), but according to the OED "farther" comes from an originally correct "farrer", which someone over-corrected to farther. So that took me an aeon to find out too, and if it turns out to be easy to find in your grammar book, then that's probably a coincidence!
This thread is probably a repeat thread, but just in case, I have a friend whose wife went to a posh girl's school and her English teacher one day said, in a voice like Joyce Grenfell's, "Remember, girls, it's LESS gin, FEWER cigarettes!"
1975 Calatayud y Gisbert, Yamaha CG131S.