Gothic is the earliest extensively recorded Germanic language that has come down to us. Knowledge of it is derived primarily from the remains of a Bible translation made in the 4th century by the Visigothic bishop Wulfila or Ulfilas, although the surviving manuscripts are not originals but later copies thought to have been written in northern Italy during the period of Ostrogothic rule (first half of the 6th century). They include considerable portions of the New Testament and a minor part of the Old Testament (Nehemiah). Other remains are scarce and include fragments of a commentary on St. John's Gospel (the so-called Skeireins), a fragment of a calendar, two deeds containing some Gothic sentences, and a 10th-century manuscript which gives the Gothic alphabet, a few Gothic words with Latin translation, and some phonetic remarks with illustrative examples.
Anyone who is seriously interested in the history of Germanic languages should have at least some knowledge of Gothic: the language sheds light on the transition from Indo-European to the various Germanic languages and gives clear understanding of their structure in general. For a thorough discussion, we refer to our digital facsimile edition of Wilhelm Streitberg's Gotisches Elementarbuch (1920). Although a little dated in some aspects, notably Streitberg's reliance on Sievers' Intonationsforschung, it's still a very useful primer.
The standard bibliography is the Bibliographia Gotica, started by Fernand Mossé in 1950, continued by James Marchand, Ernst Ebbinghaus and now Christian Petersen. A selection is available online at David Landau's Database of Selected Books and Articles (edited by Petersen). WEMSK is also an interesting starting point.
Good overviews are Peter Tunstall's Gothic Links (including references to other Old Germanic languages) and Reimar Müller's Gotisch im WWW (in German). Here is a small selection:
devoted to the study and discussion of the historical East-Germanic people known as the Goths, Visigoths and Ostrogoths. This list is concerned with all aspects of that tribe: social, cultural, historical and linguistic.