The books by South African minister and theologian, Andrew Murray (1828 – 1917), have profoundly impacted my spiritual walk. I cannot recommend them strongly enough!
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Biblehub.com is an excellent online source for Biblical research, with parallel translations, commentaries, definitions and much more. And it's all free.
Edward McKendree "EM" Bounds (1835 – 1913) was a clergyman of the Methodist Episcopal Church South and author of eleven books, nine of which focused on the subject of prayer. These are another great resource on understanding prayer.