This page is mostly just a list of websites, books, etc. that I wanted to keep in one place, mainly dealing with languages, folklore, and mythology, with some other things perhaps. The resources are not arranged by quality, but personal favorites are marked with a 
star.
Also, I would like to note that these are just things I've used or wanted to use, or books and translations I own. These are not meant to be the best of what's to be found, and I haven't read them all in their entirety yet.
- Book
- Youtube channel
- Blog
- Irish; Old Irish
- Welsh; Old Welsh
- Cornish
- Old English; Anglo-Saxon
- Norse; Nordic
- Icelandic; Old Icelandic
- - Personal favorite
Languages
Celtic
- Teach Yourself Irish (1961) by Myles Dillon and Donncha Ó Cróinín - teaches the West Munster dialect
- Learning Irish (1988) by Mícheál Ó Siadhail - teaches the Cois Fharraige dialect
- Say Something in Welsh - a pretty popular audio course with both North and South Welsh versions
- Modern Welsh: A Comprehensive Grammar by Gareth King
- Some Basic Rules of Welsh Grammar
- DIY Welsh by D. Geraint Lewis - I've never used this book, but I'd really like to some day
- An Introduction to Early Welsh (1909) by John Strachan
- A Grammar of Middle Welsh (1964) by Daniel Simon Evans
- Say Something in Cornish - the Cornish version of Say Something in Welsh
- KDL Courses from Kesva an Tavas Kernewek - introductory materials aimed at children in the sidebar
- Towards a grammar of Middle Cornish by Lauran Toorians
Germanic
- An Introduction to Old English (2002) by Richard Hogg
- Icelandic Online
- Faroese Online