The Poetic Wonder of Isaac Watts by Douglas Bond is free from Ligonier Ministries' online store in DRM-free
epub and
mobi.
Be aware that signing up in their online store adds you to their (snail mail) mailing list. In addition to sending a lot of fundraising letters itself, Ligonier also sells its list (Rand Paul apparently thinks that fans of R. C. Sproul are fertile ground). If that would annoy you, the book's also free at
Amazon.