just thought i'd give the basic details of some of the books available out there...
1) "Foundations of T'ien T'ai Philosophy: The Flowering of the Two Truths Theory in Chinese Buddhism", Paul L. Swanson.
2) "BDK English Tripitaka 107-I, III" [ both books within this volume are of value: "The Essentials of the Eight Traditions" gives a basic overview of the different sects of Japanese Buddhism including Tendai. "The Candle of the Latter Dharma" is a document written by Saicho describing the situation for those wishing to study Buddhism during Mappo].
3) "BDK English Tripitaka 97-I, II" [book two within this volume is of immense value, it is "The Collected Teachings of the Tendai Lotus School" as submitted by Gishin sama].
4) "T'ien-t'ai Buddhism and Early Madhyamika", NG Yu-Kwan.
5) "Stopping and Seeing: A Comprehensive Course in Buddhist Meditation", Thomas Cleary. [is an abridged translation of chapter 1 of the Moho Chih Kuan/ Maka Shikan].
6) "Trekking through Hell to Find Heaven; Experiencing traditional Japanese Seminary", Jion Prosser. [is a diary kept by Rev. Jion Prosser during Gyo-in on Hieizan].
7) "The Essentials of Buddhist Meditation: The essentials for practicing Calming-and-Insight & Dhyana Meditation", Bhiksu Dharmamitra. [ translation of the Xiao Zhi Guan/ Sho Shikan.
8) "The Six Dharma Gates to the Sublime", Bhiksu Dharmamitra. [translation of Zhiyi's Roku Myoho Mon].
9) "Zen is for everyone", Michael Saso. [translationf of Zhiyi's Xiao Zhi Guan or Sho Shikan-- makes a good comparison to the one released by Bhiksu Dharmamitra].
10) "Saicho: The Establishment of the Japanese Tendai School", Paul Groner.
11) "Tantric Art and Meditation: The Tendai Tradition", Michael Saso.
12) "Pure Land Buddhism: Dialogues with Ancient Masters", Amitabha Buddhist Association. [ this book is given out freely by the Amitabha Buddhist Association, Part 1 is an essay by Zhiyi, founder of Tendai entitled: 'Ten Doubts about the Pure Land'].
13) "The Great Calming and Contemplation of Chih-I: Chapter 1", Neal Arvid Donner. [This is a translation of the first chapter of the Maka Shikan, unabridged submitted by the above author as a requirement to fill a doctoral of philisophy- it is available freely on the internet somewhere although i cant quite recall where...].
14) "Profound Meaning of the Lotus Sutra", Haiyan Shen. [contains a translation of the text and commentary of Zhiyi's Fahua Xuan yi].
15) "Japanese Temple Buddhism; worldliness in a religion of renunciation", Stephen G. Covell. [provides a discussion of the dilemna of married clergy in Japanese Buddhism but focuses specifically on Tendai.
16) "Neither Monk nor Layman; Clerical marriage in modern Japanese Buddhism", Richard Jaffe. [again discusses clerical marriage from a particularly Tendai perspective.
17) "Legend and Legitimation", Jinhua Chen. [an attack on the legitimacy of many of Tendai's lineages, specifically those pertaining to Mikkyo].
18) "Ryogen and Mount Hiei", Paul Groner. [provides history of Tendai sect in and around the time of Ryogen].
19) "Tendai Gongyo", Rev. Jion Prosser.
20) "Evil and/or the Good: Omnicentrism, intersubjectivity and value paradox in Tiantai Thought", Brook Ziporyn.
21) "The Kurikara Dairyu Esoteric Ritual", Rev. Jion Prosser.
22) "Being and Ambiguity: Philosophical Experiments with Tiantai Buddhism", Brook Zipoyrin.
...at the moment those are the only ones i can think of, perhaps others with more knowledge will be able to add others to this humble list of mine
May this list bring much merit and help others along their paths...