Half-hours with the Telescope

“Half-hours with the Telescope: Being a Popular Guide to the Use of the Telescope as a Means of Amusement and Instruction” by Richard A. Proctor has a lot of detail about telescopes such as the lens and focal length which increases an amateur stargazer's understanding to make his own telescope. While this book was written well before modern telescope models were on the market, it's still an essential resource as all versions use the same basic construction and require the same understanding.