Sabriel → Lirael → Abhorsen is the core trilogy. After that: Goldenhand (direct sequel), then the prequels Clariel and Terciel and Elinor can be read in any order. Publication order (Sabriel → Lirael → Abhorsen → Clariel → Goldenhand) is recommended over chronological.
Clariel is set 600 years before Sabriel and follows a different protagonist. It can technically be read standalone but is better appreciated after the main trilogy since it recontextualises events and magic from those books.
The magic in the Old Kingdom is split between Charter Magic (structured, learnable runes drawn from a great Charter) and Free Magic (dangerous, chaotic, tied to entities). The Abhorsen's necromantic bells — each with a specific function — are one of the most original magic systems in fantasy.
