I've learned a lot from zaphaudio.com. Also, a lot of people recommend Vance Dickason's Loudspeaker Design Cookbook. Also check out various forum FAQs. Ask questions on the forum. Attend a diy meet if you can. Helps to have a background in science and math. Build a speaker design and reverse engineer it. Build one from a designer with an open line of communication and ask them all about it. Ask them how they did it.