There's a series of books available at Pete Bland Sports...

Training For Uphill Running (or similarly titled)
Training For Downhill Running

In the same series there's one more tailored to Mountain Marathon type events. I'd imagine that one, if not all of them, would be of interest.

All the books i mentioned are on here...

https://www.peteblandsports.co.uk/de...oks_d01719.htm