A classical reference is **Hypothèse du Continu** by Waclaw Sierpiński (1934), available through the Virtual Library of Science as part of the series *Mathematical Monographs* of the Institute of Mathematics of the Polish Academy of Sciences.

Sierpiński discusses equivalences and consequences. The statements covered include examples from set theory, combinatorics, analysis, and algebra. Most of the consequences he did not show equivalent were found later (mainly by Martin, Solovay, and Kunen) to be strictly weaker in that they follow from Martin's Axiom, and some are discussed in the original Martin-Solovay paper.

(In fact, the discovery of Martin's Axiom and the subsequent research on cardinal characteristics of the continuum helped clarify what the role of CH is in many classical arguments, and nowadays results that classically would be stated as consequences of CH are stated as consequences of some equality between cardinal characteristics. See the articles by Blass and Bartoszyński on the **Handbook of Set Theory**.)

Of course, many equivalents were found after 1934. For example:

- Around 1943, Erdős and Kakutani proved that CH is equivalent to there being countably many Hamel bases whose union is $\mathbb R\setminus\{0\}$.
- In the early 60s, Erdős found a nice equivalent in terms of analytic functions (see Chapter 17 in Aigner-Ziegler
**Proofs from THE BOOK**).
- Quite recently, Zoli proved that CH is equivalent to the transcendental reals being the union of countably many transcendence bases.

I do not know of an encyclopedic work updating Sierpiński's monograph. Most recent work on CH centers on what Stevo Todorcevic calls *Combinatorial Dichotomies in Set Theory*. It turns out that for quite a few statements, CH proves a "nonclassification" result, while strong forcing axioms (such as PFA) prove strong "classifications". For example, J. Moore proved that there is a 5-element basis for the uncountable linear orders if PFA holds, while Sierpiński showed that CH gives us $2^{\aleph_1}$ non-isomorphic uncountable dense sets of reals, none of which embeds into another in an order-preserving fashion.

Though not specifically concerned with CH and its equivalences, you may find interesting Steprans's *History of the Continuum in the Twentieth Century*.

Another recent line of study on CH centers on the role of choice. Propositions equivalent to CH in ZFC may have wildly different truth values if choice is not assumed. For example, under determinacy, CH is true in the sense that every set of reals is either countable or of the same size as the reals. However, it is also false in the sense that $\aleph_1\not\le|\mathbb R|$, and that there is a surjection from $\mathbb R$ onto $\aleph_2$.