The main reason knowledge systems did not become ubiquitous (compared with relational database systems) is the knowledge acquisition bottleneck: it's hard to build systems that model and act on knowledge of human experts. Tom worked on systems to help knowledge systems build up their knowledge by having them interact with experts in the context of solving problems.

Tom built systems that learned from users by allowing them to demonstrate what they knew, and then explain why it was the right thing to do.

Later he applied these principle to help human organizations capture their knowledge as they design things, so called design rationale capture.

• Thomas R. Gruber (1988). Acquiring strategic knowledge from experts. International Journal of Man-Machine Studies, Volume 29 , Issue 5 (November 1988), pp. 579-597. Reprinted in The Foundations of Knowledge Acquisition, 1990, pp. 115-133, Academic Press, ISBN:0-12-115922-1.
• Thomas R. Gruber (1989). Automated Knowledge Acquisition for Strategic Knowledge. Machine Learning, Volume 4 , Issue 3-4 (December 1989), pp. 293 - 336.
• Thomas R. Gruber (1991). Interactive Acquisition of Justifications: Learning "Why" by Being Told "What." IEEE Expert, 6(4): 65-75, August 1991.
• Thomas R. Gruber and Daniel M Russell. (1992). Generative design rationale: Beyond the record and replay paradigm. In T. Moran and J. H. Carroll (Eds.), Design Rationale: Concepts, Techniques, and Use. Lawrence Erlbaum Associates, 1995, pp. 323 - 349. ISBN:0-8058-1567-8.
  Originally written in 1992, on the web in 1993, in print in 1995!