Reddit mentions of Scale Chord Relationships: A Guide to Knowing What Notes to Play - and Why!

Okay so for teaching yourself music theory I believe this was the book I used https://www.amazon.com/Practical-Theory-Complete-Spiral-Bound-Book/dp/0882842250 although I can't find my copy to be %100 sure. It was super cut and dry so maybe boring but I think of music theory as like a sudoku puzzle and find it fun. Basically this should teach you basic stuff like the major and minor keys and how chords are built. Learning the fundamentals is super important to really understanding theory in a meaningful way.

Once you get the fundamentals down learning roman numeral analysis will help you understand how songs are built and why chords work the way they do. For example in Casey Jones I know that the D major chord is V/V which is called a secondary dominant which means that it is acting like a dominant V chord pulling to G so I know when soloing on it that the F# in that D major chord is super important because it is the 7th or G major and especially since F# isn't in the key of C it is a note that causes a lot of tension and is just begging to be resolved up a half-step to G. Now none of that probably made sense but its just to show why learning the roman numeral analysis of Bach and Mozart can be helpful to understanding the Grateful Dead.

Learning Roman Numeral analysis on your own maybe tricky so like I said in my other comment if you can take a music theory class as an elective at your school that would be best. If you can't though, you could just buy the textbook yourself like Clendinning/Marvin's Musician's Guide to Theory and Analysis or look through this thread for suggestions https://www.reddit.com/r/musictheory/comments/pse4l/beginners_resources_for_the_sidebar/?st=jf744ciz&sh=fe9dd4bd .
There are a lot of videos out there such as https://www.youtube.com/watch?v=ICDPWP6HUbk&list=PLw9t0oA3fHkxx1PgYpiXrMUPXaOiwh6KU but you have to poke around YouTube to find ones that work for you.

And than for applying this stuff to guitar you'll want to learn all the notes on the guitar. To do this sight reading whatever you can get your hands on (I go to my local music store and buy flute music), classical guitar music, jazz or folk tunes, or just googling sheet music for whatever songs you want to learn and playing this music in different areas of the guitar. Even sight reading a simple melody can be hard in the 9th position if you aren't used to it. If you are having trouble learning treble clef pick up one of the Mel Bay guitar books https://www.amazon.com/Mel-Bays-Modern-Guitar-Method/dp/0871663546 .
Another good way to learn the notes on the guitar is to play scales and arpeggios in all positions and than say each note out loud as you are playing it. Obviously you will first need to learn your scales and arpeggios but once you do play them on guitar or to make that work book I recommended less boring play your guitar as you go through it.

And lastly the most practical thing you are looking for is a Chord-Scale relationship book like this one https://www.amazon.com/Scale-Chord-Relationships-Knowing-Educational/dp/0634019945 I've never owned this book but it looks alright and any chord-scale theory book should help you.
Chord-Scale theory is basically like "on this chord you play this scale" and it's a terrible way to learn jazz and in general it over simplifies everything and I hate it lol buuutttt I do think it will help you with Grateful Dead stuff. It's just very surface level stuff so I'm putting it last to hopefully encourage you to still learn the more boring classical stuff, because it will pay off in the long run to learn how analyze a Mozart piece. However, if you just kinda wanna start getting into it I think a Chord-Scale Theory book will help you out the most quickly.

Also musictheory.net is a good website to check out to help with this stuff. Good luck and let me know if you have any questions!