Accents: A Manual for Actors- Revised and Expanded Edition https://www.amazon.com/dp/087910967X/ref=cm_sw_r_apan_glt_fabc_QB63PQWXDKHRKR4XPM2J?_encoding=UTF8&psc=1
That's a book I've used. It helps being able to see it, read it and hear it. It either comes with CDs or a link for audio samples of every Accents in the book. A usefull tool.
I am not an auditory learner, so I have a huge amount of difficulty with accents. I primarily differentiate my NPCs by their word choices, not by accents, though I can pull off a Texas accent pretty well. However, this book has helped me some, better than nothing at least. I'm working on a couple of accents and am hoping to eventually expand my repertoire, but at the moment all I can do particularly well is my own American accent (don't ask me for the region, I moved around enough as a kid that it's a mutt accent with elements of the places I've lived and my parents' accents), a Texan accent, a semi-passable "Southern belle" accent, and a decent Welsh accent.
Though the Welsh I mostly learned from watching scenes in Torchwood repeatedly and repeating all of Merrill's lines from Dragon Age 2.
I'm working on a Manchester accent and attempting to learn whatever Tali from Mass Effect's accent is.
Accents are funny. It's all oral placement and muscle memory. There are books that are helpful. This one is amazing: http://www.amazon.com/Accents-Actors--Revised-Expanded-Edition/dp/087910967X/ref=sr_1_1?ie=UTF8&qid=1388396719&sr=8-1&keywords=learning+accents I used it to learn Irish for a play about 8 years ago. It also just requires the ability to listen really well and mimic phrases. It all starts with a single word, then a phrase, then a sentence, then a thought. There's almost a musicality, rhythm, and playfulness when discovering an accent. I usually zone in on the attitude and the rest follows suit. I love the BBC and other international channels. They help a lot. Recalling accents is easy if you have trigger phrases that you can say at anytime that pull you back into the accent. I've gone into that here already on another Q. Take a peek down there to get more ideas.
I hope that helps!
Here's a database of speaking audio separated by region of origin, and labeled with their age and education: https://www.dialectsarchive.com/
And here's "Accents: A Manual for Actors" (Amazon link), a guide to speaking English so you sound like you're from anywhere, including sound changes, pitch, and stress (maybe vocabulary? I can't remember)