A very clear but short lamrim text is "The Path to Enlightenment," which is the 14th Dalai Lama's commentary on "Essence of Refined Gold," by Sonam Gyatso, the third Dalai Lama. http://www.amazon.com/Path-Enlightenment-Dalai-Lama/dp/1559390328/ref=cm_lmf_tit_24
As mentioned, Phabongkhapa's "Liberation in the Palm of Your Hand" is another very good text in the same genre. It is longer, but it is well worth the read.
Tsongkhapa, the progenitor of the Gelugpas, was very prolific; however, his works are very technical and, in my opinion, not the best option for someone looking for an introduction.
Three other books I would suggest are Geshe Sonam Rinchen's commentary on Atisha's "Lamp for the Path to Enlightenment" as well as two more books from the current Dalai Lama, "For the Benefit of All Beings" and "Practicing Wisdom." Atisha was a Kadampa but his seminal lamrim work is a foundational Gelug text, and Geshe Sonam Rinchen is a Gelugpa as well. The two other books from the Dalai Lama should really be a single volume as they comprise a single, short commentary on Shantideva's "Way of the Bodhisattva."http://www.amazon.com/Atishas-Enlightenment-Geshe-Sonam-Rinchen/dp/1559390824/ref=cm_lmf_tit_9_russss0http://www.amazon.com/Benefit-All-Beings-Commentary-Bodhisattva/dp/1590306937/ref=cm_lmf_tit_18_russss0http://www.amazon.com/Practicing-Wisdom-Holiness-Dalai-Lama/dp/0861711823/ref=cm_lmf_tit_19_russss0
All these works focus mainly on the sutrayana. I am not as familiar with good introductory books on tantra within the Gelug school.