Here are a few more recommendations:

The Still Hour, or Communion With God in Prayer -- Austin Phelps
A Call to Prayer -- J.C. Ryle
Time for Secret Prayer -- Thomas Brooks
The Secret Key to Heaven -- The Vital Importance of Private Prayer -- Thomas Brooks
The Closet: or, Secret Prayer Successfully Managed -- Samuel Lee (see also The Bible and the Closet -- Thomas Watson and Samuel Lee)
Lord, Teach Us to Pray -- Alexander Whyte
The Christian's Daily Walk -- Henry Scudder
-- Robert Bolton
The Most High a Prayer-Hearing God -- Jonathan Edwards
The Necessity of Secret Prayer -- Thomas Boston
For What, and How We are to Pray -- Thomas Boston
The Lord Prayer, its Spirit and its Teaching -- Octavius Winslow
The Practice of Piety -- Lewis Bayly
The Christian's Reasonable Service -- Wilhelmus a Brakel

__________________
Andrew Myers
Husband of Jessica, Father of Jackson, Katie and Samuel
Member, Presbyterian Reformed Church of Northern Virginia
Warrenton, VA USA
Editor, The Matthew Poole Project

"Let your Morning Thoughts, and your last Evening Thoughts, be what shall become of you to all Eternity." -- Matthew Poole