For learning preparation methods, a few resources stand out:
Medical Herbalism is one of the best references for understanding why different herbs are prepared in different ways. It includes infusions, decoctions, tinctures, syrups, oils, poultices, compresses, and formulation principles.
The Art & Science of Herbal Medicine has a beginner-friendly section devoted to preparation methods and building an herbal toolkit.
The Holistic Home Apothecary Book walks through harvesting, drying, storing, teas, tinctures, oils, salves, syrups, compresses, and other home preparations.
The Everything Guide to Herbal Remedies provides a practical overview of common herbal preparations and how they are used.