The most recent and authoritative work on Indian Epigraphy is that by Salomon (1998). This is a first reference for inscriptions in general, and supersedes Sircar's (1965) previous summary. Still essential as a reference is Sircar's (1966) Epigraphical Glossary, which is the standard reference. More specific are...
On Paleography, the only detailed analysis of Kushan inscriptions is by Lohuizen-de Leeuw (1949), though readers can also consult Dani's (1968) contribution to the 1960 conference on the date of Kanishka. The most comprehensive work on paleography is by Dani (1986) who covers a broad range of inscriptions, and is scathing of Lohuizen's approach. More specific are Rajgor (2000) who is concerned mostly with the inscriptions of the Western Ksatraps, Sharma (2002) who has looked at Brahmi the the north-west of Mathura, and included coins and documents in his analysis (which is thorough but rather poorly presented), and Verma (1998) which covers the Imperial Gupta inscriptions in detail.
Essential if you are interested in the inscriptional evidence is a collection of those inscriptions. The most accessible is that by Shrava (1993) which collects together over a hundred of the dated Brahmi inscriptions and includes images of most. Also of interest is Luders Mathura Inscriptions, edited after his death by Klaus Janert. But the vast majority of inscriptions are still scattered across journals. Readers interested in pursuing particular inscriptions can consult the list of preferred readings on this site. This is the most comprehensive and up to date listing of inscriptions relating (broadly) to the Kushan period available anywhere.
http://indepigr.narod.ru/index_1.htm, includes pdf versions of Luders lists of Brahmi inscriptions, originally published in Epigraphia Indica 10. A very valuable supplement to the list on this site.
http://www.ancientscripts.com/brahmi.html, includes a chart of Brahmi characters
http://www.cs.colostate.edu/~malaiya/scripts.html, on the scripts of India includes links to a variety of articles and sites on the Brahmi script which may be of interest.