Classical writers such as Tacitus (Germania) and Caesar (De Bello Gallico) are the earliest sources but they were outsiders so what they can tell us has thus a limited value. Caesar in particular presents a very primitive view of ancient Teutonic religion, stating:
"The customs of the Germans are very different from those of the Gauls. They have no druids to preside over religious matters, nor do they concern themselves with sacrifices. The only things which they count as gods are things they can see and which clearly benefit them, for example, the Sun, Vulcan, and the Moon. They have not even heard rumours of any others." (6.21)How different this perception is compared to that of Tacitus writing over a century later in the first century CE! We know that the Germans DID have a priesthood who exerted a great deal of influence and we also know that sacrifices were made to the Gods. Clearly the Germanic tribes honoured the deities behind the sun, moon and fire (Vulcan) but were not limited to these. Tacitus mentions deities not referred to in the Eddas such as Tuisto, Mannus, Nerthus and the Alcis.
Viktor Rydberg however did an excellent job in creating an exposition in epic form of all the available material in his two volume Teutonic Mythology which has been published in the form of 3 books, the second volume being printed in 2 parts. Rydberg's Our Fathers' Godsaga: Retold for the Young ironically is actually better suited to adults! It is a handy single volume retelling of the material in Teutonic Mythology. I recommend reading this volume as an introduction to the other 3 books.
Jacob Grimm's Teutonic Mythology, available in 4 volumes contains material that is additional to that found in the Scandinavian material and focuses more in what can be gleaned from continental Germania, utilising folklore, folksongs, ancient spells, herblore, mediaeval manuscripts and place name etymology. Those seeking to gain a more primitive, a more Germanic or a more German understanding of our ancient beliefs and practices would do well to study these volumes.
It is important to study as much as we can in order that we have a sound basis on which to rebuild our ancient heritage and religion and this takes many years. Finding a reliable mentor, teacher or organisation will help to guide the novice but it is imperative that book learning does not replace having a deep relationship with the Gods and this comes with the practice of regular rites and meditation.