The Authoritative 18th Century Guide to Heartfelt Prayer and Inspired Service of Hashem
She'arim 8-9
Published close to 250 years ago by Rav Alexander Ziskind of Horodna, Sefer Yesod VeShoresh HaAvodah outlines the kavanos, the intentions we should have before performing any mitzvah. It also brilliantly describes the intensity and kavanos we should have in our prayers, and the deeper meaning of specific tefillos.