Where can I learn about HEMA shields?

The Bolognese sources (1500s) contain information on fighting with both buckler and larger shields, but more importantly, cape. Achille Marozzo has large sections on both. Antonio Manciolino mainly deals with buckler. Giovanni dall'Agocchie is a good read too.

Have a look at wiktenauer, several translations can be found for free.

Some books discussing Long Shield are Codex Wallerstein, Die Blume des Kampfes, Kunste Zu Ritterlicher Were, Paulus Kal's fencing manual, Paulus Hector Mair's compilation (largely re-illustrations of early fencing manuals), and Hans Talhoffer's MS Thott.290.2º and MS XIX.17-3.

Those links will bring you to illustrations and translations of the text, but admittedly, a lot of them are from an era where an illustration and a sentence or two was provided. I don't know any of the more modern treatises.