I just finished Thomas a Kempis' landmark work, "The Imitation of Christ", which is often called the world's best-selling book, second only to the Bible. Given that it was published in 1418, that's not too surprising, and the book is clearly a product of its age, although it does remain contemporary for today's Catholic (and Christian...but they may have to skip the parts on devotion to the Eucharist).
The work itself is a series of short chapters divided into four sections: 1. Spiritual Admonitions; 2. Admonitions pertaining to inward things; 3. Internal Consolation; and 4. A Devout Exhortation to the Holy Communion. When I say these chapters are short, I mean it. They're more like meditations, most only a page or two long, that make it perfect for daily reading. It can be difficult to read many chapters in one sitting, because the themes tend to run together or become repetitive, so I would recommend reading it daily, or choosing a day of the week and reading one or two and then spending 15-20 meditating on what you've read. All of his entries are chock-full of theological insight and inquiry, so they take some digesting in order to get the full effect.
The book focuses on living only for God and renouncing things of this world as much as possible, even other human relationships, making sure that every relationship you have and everything you do only brings you closer to God and Heaven. It's a bit sparse for our modern minds to comprehend, and can seem somewhat tough to follow--it's a very asture life he's advocating here. It's also a life of great self-sacrifice and self-denial, training yourself to only want Jesus and to follow His ways by becoming purer in prayer, life, thoughts, and practice, which involves sort of stripping yourself of the world and worldy cares and vices. Like I said, it can seem a little medieval (which is fitting, given when it was written), but don't let that stop you from reading it. It still contains great pieces of religious advice and thoughts on prayer and Eucharistic devotion.
Some passages are written in the voice of Jesus ("the Beloved") and some are written in the voice of A Kempis inquiring after Jesus ("the follower of the Beloved" or something like that...depends on the translation). Those dialogues, more toward the end of the book, are interesting and insightful, as is the book on Eucharistic devotion.
I just mentioned translation--I have one from 1900 (Barnes and Noble Library of Essential Reading $7.95) that is loaded with thees and thous and wiltnots and stuff like that. If you, like me, are familiar and fond of Shakespeare and high English dialect and translation, then this is OK for you. If you like a clearer, more modern translation, this isn't. I'm sure your local library, bookstore, or the omnipresent Amazon.com will be able to help you track down a more modern version of the text.
This would make good reading for older teens (that are spiritually and intellectually mature) and adults. Parts of it can be hard sledding, so don't give up if he loses you at some points. I found it helpful to keep a pencil handy and mark the sections that were particularly helpful or beautiful to me, in order to keep me focused and on-track. This would also be a useful trick in guiding meditations.
I would also recommend A Kempis' "The Passion of Christ, According to the Four Evangelists." It is also written in a meditation-style (v. short chapters) and I read it every Friday as my meditation for the day. It is extremly appropriate for Lenten devotion, and is less on the 'thees' and 'thous'--I have the translation for Ignatius Press, which is very good; beautiful but clean and stylish. (But everything from Ignatius Press is good) Both these books would make wonderful gifts for others or yourself and will enhance your spiritual life immensely as you meditate on the ideas he sets forth.