You show me a professional musician, rock, jazz, country, or bluegrass and I'll show you someone that spent their formative years bent over a half speed tape recorder (or if they are old enough... a variable speed record player).
You're thinking "Wait a minute, what does this have to do with learning by ear?".
In order to learn the melodies, leads, and chords from a recording, it's usually necessary to slow things down. Some people that are unusually gifted and have incredibly good ears can do more than most of us by listening at full speed, but even those folks can't  figure out faster solos without some kind of electronic aid. For the rest of us, it's even hard to figure out slow music without slowing things down. Why is this? Well, most of us can match a pitch with our instrument or voice if that's the only thing we hear. However, when lot's of notes are put back to back, we get confused and can't lock in on the individual pitches. If we can slow the music down enough, we can work our way through the piece one note at a time until we get the music.
In the past, this was done with half speed tape recorders. The problem here is that as you slow the music down, the pitch drops. By the time you get to 1/2 speed, it has already dropped by one octave. So you see it's not practical to go much slower. The drop in pitch makes it much harder for us to figure things out and for most of us, 1/2 speed is just not slow enough. In fact, I do transcribing for magazines and frequently require 1/10 speed to figure out certain passages exactly.
This is where our digital slowdown technology comes into play. We can slow the music down as far as we want, but keeping the same original pitch.
You might be thinking, "Wow, figuring out those things note by note seems like an awful lot of trouble. Can't I just buy a book and skip the figuring out on my own?"
The answer is, yes you can buy the book and yes that book will most likely just gather dust on your bookshelf.
Experience has shown that you need to be involved in the process by the act of listening that goes along with figuring things out yourself.
Without getting into a giant essay on this, I'll just say that figuring out the music by yourself is the best, but trying to figure it out and consulting with a good book of "Transcriptions" can also be very good.
Some people prefer to just learn from the Transcription books but play along with the original recording at a slower speed in order to get all the nuance of the original