These are the rule books for Classic Traveller.
Books 1-3 are core books of the Traveller system, though core rules were also available in other forms such as the start set and the Traveller Book which are not part of this series.
This series, along with the adventures and supplements in the same form, are sometimes called the little black books. They were all published as plain black covers with a single color writing on them.
From an Overview of Traveller Material appendix in Book 0: An Introduction to Traveller:
Books are compendiums of rules, and range from 48 to 56 pages in length.