I'm very interested in Tefillin and I was wondering if anyone knew of any traditional sources which are either dedicated to or devote serious attention to the meaning of the mitzvah & it's status as a spiritual practice. Not just a hilchot tefillin book, I've got that. I've also read Aryeh Kaplan's book, which is pretty good but the style of which leaves something to be desired. Anyone know of a sefer or two of this kind?
I highly recommend this sefer on teffilin alot of interesting information with deep concepts of teffilin. The Inside story of teffilin by Rabbi Moshe Shlomo Emanuel.
I just remembered that I read and liked this short book which perfectly meets your description (available on Kindle for 1$ or in paperback)
From one of the amazon reviews
Like Rabbi Aryeh Kaplan's NCSY booklet on Tefillin, written more than three decades ago, this little gem, "hot off the press," communicates in simple language sophisticated spiritual concepts related to the mitzvah of Tefillin. Many boys prepare for the milestone event of their bar mitzvah by studying the basic laws of Tefillin (whether more or less), but often the profound meaning of what they are doing is barely touched upon. Rabbi Morrison's book, which presents Rav Kook's insights in easily understood modern language, remedies this problem. No doubt many older readers would find it equally enlightening.