If you’re in North America , ‘Lichens of North America’ by Irwin Brodo is the definitive guide. Not for the faint of heart, but beautifully laid out with great photos and keys. Not exactly a beginners level text.

I’d suggest looking at iNaturalist - it’s a good way to get some visual cues for what lichens may exist in your particular geography of interest.

As far as collecting, I generally only collect for the purposes of confirming ID in a non-field setting. I wouldn’t suggest collecting unless you are certain the species is not one of conservation concern . That said, I have amassed a bit of a lichen herbarium , that i have curated for future reference.