There are some really good books out there, but I think a great way to start is to take a recipe you like and start experimenting with it. Switch up the hops, switch up the mash temperature, boil time, gravity, etc. Each change will make a different beer, although some will have a bigger effect than others. But starting with a good baseline beer is a good way to know you’ll have something decent.
Also, a bitterness unit to gravity unit chart (BU/GU chart) and some simple brewing software to calculate starting gravity and IBUs are invaluable. Start with this chart, and pick a starting gravity. Then decide if you want a more bitter or malty beer (like an IPA versus a British bitter), and come up with the IBUs through your hopping schedule and brewing software to match that BU/GU ratio. More hop flavor means later hop additions, and cleaner bitter beers with just 60 and maybe 30 minute additions. Ok to keep it simple.
And smash beers don’t have to be simple. I really like this one barleywine recipe I make with a starting gravity of 1.120, all EKG hops, and a three hour boil. The final beer comes out a deep amber color, and an incredible amount of malt complexity from the long boil. But don’t be afraid to keep it simple, see what you get, and adjust it from there. Small changes almost guarantees that you’ll have something enjoyable in the end.