Books
How To Make It In The New Music Business - Ari Herstand (Awesome but US focussed)
Podcasts
Resources
Youtube Channels
https://www.youtube.com/channel/UCLtD67ljlaeXQMV4sb-YzNA - Holistic Songwriting
https://www.youtube.com/channel/UCexjApxCp3sCEHZu4M33iOg - Burstimo Music Marketing
Blogs
https://music3point0.com/ - Bobby Owsinski
Gigbag Essentials - Guitarists
General
Hearing Protection
https://www.hearformusicians.org.uk/ - In the UK with this link you can apply to have a hearing test and custom moulded in ear protection worth £280 for £40, £30 if you are an MU member.
ER20's - Flat, less comfortable than custom moulded but sound the same. £20
Vocal Specific
XLR Van Damme star quad cable, Neutrik connectors multiple colours No one likes touching sticky venue cables, this cable above can be in your chosen colour to stand out from the crowd and is made from the best connectors and cable. Star quad cable also makes it less prone to interference and if one of the connectors inside fails the show will still go on as each core has a backup line.
Although Venues often provide microphones it is one of the easiest ways to get ill sharing a live microphone it's well worth getting your own!
SE V7 Microphone - One of the best currrent live microphones
Shure SM58 - The Classic
Shure Super 55 - The old school retro mic (same internals as the 58)
Guitar & Bass Specific
Spare tubes for your amp, Strings, batteries and pics are essential. Everyone has their own preferences and needs but here are a few things that I'd also take in my kit bag.
Mono M80 dual guitar gig bag - carry your main and back up to your gig in one case.
Servisol Super 10 - Clean your scratchy pots guitarists
Big Bends Nut Sauce - Sort your tuning stability and stop string breaks
GHS Fast Fret - Some people swear by this to keep strings clean some people arent fans, I love it.
Gruv Gear Fretwraps string muters - If you are playing lead these can massively help.
G7 Capo - by far the best capo in my opinion.
Drum Specific
Moon Gel - Dampen your drums to change the length of the hit
SlapKlatz - I often prefer this to moongel on drums, the different sizes really help
Laptop
Radial SB - 5 Laptop - Want to plug your laptop in with no buzz without an interface?