Many professors write their own textbooks. As a hobbyist, I suggest you learn color theory, space planning and then familiarize yourself with the fundamentals of various styles, eg mid century modern, French salon, southwestern, colonial, modern loft, contemporary, Tuscan, ca beach, wine country, farmhouse, country western, etc. You will also need to learn the basic design elements of arranging pieces (magic 3) plus lighting theory and scale of design. How to combine textures. Know how to read a blueprint and draft a design to 1/4 inch scale. Elements of Design is often used as a textbook for designers. It's expensive and weighs a ton but covers all the basic classic designs.

In other words, interior design is a trained profession that takes years of study and practice. Interior decorating is less rigorous and those professionals usually work for retail stores and rely on the products offered by their employers. Some colleges offer interior design classes and degrees through their Home Economics Departments too. The most experienced Interior designers have earned their ASID designation, which requires quite a few years in business.

Thank you for your lengthy reply!!!! I really appreciate it. Could you tell me the author of the book you mentioned? ("Elements of Design")

Thank you so much!