The very best, simple and approachable explanation about the physical properties of lumber/wood is Hoadley
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Understanding Wood: A Craftsman's Guide to Wood Technology, Book
Please make this a sticky. It will help every miller of wood to make the best of his/her lumber and will answer most questions about milling/drying/utilization of lumber. Highly recommended!
Zoom
Understanding Wood: A Craftsman's Guide to Wood Technology, Book
Please make this a sticky. It will help every miller of wood to make the best of his/her lumber and will answer most questions about milling/drying/utilization of lumber. Highly recommended!