https://smile.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon/dp/1937715221/
I like this book, good info on the technical details. Nothing will help as much as time on your feet though, so get out there. I had the same downhill issues (still do to a degree) and I did Tiger Claw VERTual in May on an insanely steep trail (1800 feet in a mile) which ended up helping a good bit.
Poles help more than you'd think, they saved at least one fall for me on that run. And as others said, keep your eyes out in front, not looking at your feet.
Hal Koerner's Field Guide to Ultrarunning (https://www.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon/dp/1937715221) is solid. I bought it as I was getting interested in ultras, and I've come back to it a lot over the past two years. They might have it at your local public library if you want to sample before you purchase.
Good luck on your 50k!
I too agree with /u/DieRunning. I would only add you could always pick up Hal Korners book, Field Guide to Ultrarunning. He gives some quick tips along with two training plans.
http://www.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon/dp/1937715221
Hal Korner's Field Guide to Ultrarunning has some shorter plans for 50k up to 100 miles. I used this book as the basis for my first 100-mile training. ( https://www.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon/dp/1937715221)
Also, Krissy Moehl's Running Your First Ultra has some good, long-term plans. ( https://www.amazon.com/Running-Your-First-Ultra-Customizable/dp/1624141420/ref=pd_sbs_14_img_0/135-5115591-8160359?_encoding=UTF8&pd_rd_i=1624141420&pd_rd_r=062b53c6-91ad-4cc2-a0c6-9225587ac7ea&pd_rd_w=jBSfT&pd_rd_wg=aenPJ&pf_rd_p=5cfcfe89-300f-47d2-b1ad-a4e27203a02a&pf_rd_r=HAZJPT342BYJYTMBPYP9&psc=1&refRID=HAZJPT342BYJYTMBPYP9)
Also, I can really recommend Hal Koerners book Field Guide to Ultra: https://www.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon/dp/1937715221
I'm using it to train for my first 100miler right now
I haven't read it, but I would check this one out: https://www.amazon.ca/Koerners-Field-Guide-Ultrarunning-Ultramarathon/dp/1937715221
I have read "Daniels' Running Formula" which is very very good: https://www.amazon.ca/Daniels-Running-Formula-3rd-Jack/dp/1450431836/ref=sr_1_sc_1?s=books&ie=UTF8&qid=1469153523&sr=1-1-spell&keywords=Daniels+runnnig
I also recommend "Advanced Marathoning" which is a better all purpose "how to be a runner" book than Daniels, but a bit less scientific.
Both have training plans for beginners. And like people mentioned: getting good quickly is easy... it's not getting hurt that is tough!