A book that I repeatedly return to is Christ Mystical by Joseph Hall (1647).
I found a very rough copy last year, and it has been a constant companion, particularly on trips up to Newcastle.
The little I know of Joseph Hall comes from wikipedia.
The main point of the book is union with Christ: The kind, the resemblance, the certainty, the privilege and benefits of this union.
Here are my favourite bits:
This last section is of particular beauty and relevance for me. On the days when I feel distant from Christ, I remind myself that He is always present. I remind myself of the wonder of His presence in the Holy Eucharist.