The following list of references, by no means complete, is meant to provide people interested in learning more about the unifying theory with some indications of materials which can be useful for acquiring the relevant technical background or deepening their understanding.

A general reference giving a non-technical introduction to the theory of toposes as 'bridges' is provided by the article "The Theory of Topos-Theoretic ‘Bridges’- A Conceptual Introduction'" (Glass Bead Journal).

**Self-contained introductory text**: click here
for a draft of the first two chapters of my book "*Theories,
Sites, Toposes: Relating and studying mathematical theories through
topos-theoretic 'bridges'"* (Oxford University Press, 2017).
This 64 pages text is a self-contained introduction to toposes,
categorical logic and the 'bridge' technique requiring only a basic
familiarity with category theory.

The book "Topoi: The Categorial Analysis of Logic" by R. Goldblatt is a very gently-paced and self-contained introduction to Category Theory and the topos-theoretic approach to Logic, so it is particularly suitable for beginners who are not already familiar with the language of Category Theory.

As a first introduction to Topos Theory for people with a basic background in Category Theory, we recommend the book by S. Mac Lane and I. Moerdijk "Sheaves in Geometry and Logic".

The book "Sketches
of an Elephant: a Topos Theory Compendium" in two volumes (the third
volume of the trilogy is currenly being written) is the fundamental
encyclopedic reference on Topos Theory. Even if this *magnum opus*
is not intended as an introduction to Topos Theory and primarily aimed
at specialists in the subject, some parts of it do not require an
advanced background to be understood; in particular, we recommend Part
D1-3 of the second volume as a self-contained introduction to
Categorical Logic and the theory of classifying toposes.

The fundamental work in which the concept of (Grothendieck) topos was originally introduced is "Théorie des topos et cohomologie étale des schémas (SGA 4)" by M. Artin, A. Grothendieck and J.-L. Verdier. This influential masterpiece is still very much cited today, and is a must-read for anyone seriously interested in Topos Theory and its relationships with Algebraic Geometry.

A general description of the unifying methodologies is provided by this paper, while specific applications of these abstract techniques in different mathematical fields can be found in most of my papers (cf. this section for a list of notable examples and relevant references).

The book From a Geometrical Point of View by J.-P. Marquis is a philosophical study of the history and foundations of Category Theory, shedding light in particular on the nature of Topos Theory and its role within the mathematical sciences.

Finally, as a very general treatment of spectra (of first-order mathematical structures) from the perspective of classifying toposes, we recommend the paper "The Bicategory of Topoi, and Spectra" by J. C. Cole. The paper "Localisation, spectra and sheaf representation" by M. Coste provides more explicit descriptions of Cole's spectra and applications to sheaf representations of models of essentially algebraic theories.