These factors in general may be divided into traditional (land, labor, capital) and modern (information, knowledge, human capital). Also other classifications are possible, and some new factors of localization could be proposed.
Here is a list of references that might be useful:
Baldwin, Richard E. and Toshihiro Okubo. 2006. Heterogeneous firms, agglomeration and economic geography: spatial selection and sorting. Journal of Economic Geography 6(3):323–346.
Combes, Pierre-Philippe, Gilles Duranton, Laurent Gobillon, Diego Puga, and Sébastien Roux. 2009. The productivity advantages of large cities: Distinguishing agglomeration from firm selection. Discussion Paper 7191, Centre for Economic Policy Research.
Combes, Pierre-Philippe, Gilles Duranton, Laurent Gobillon, and Sébastien Roux. 2010. Estimating agglomeration effects with history, geology, and worker fixed-effects. In Edward L. Glaeser (ed.) Agglomeration Economics. Chicago, il: Chicago University Press.
Dekle, Robert and Jonathan Eaton. 1999. Agglomeration and land rents: Evidence from the prefectures. Journal of Urban Economics 46(2):200–214.
Desmet, Klaus and Marcel Fafchamps. 2005. Changes in the spatial concentration of employment across US counties: A sectoral analysis 1972–2000. Journal of Economic Geography 5(3):261–284.
Duranton, Gilles and Diego Puga. 2004. Micro-foundations of urban agglomeration economies. In Vernon Henderson and Jacques-François Thisse (eds.) Handbook of Regional and Urban Economics, volume 4. Amsterdam: North-Holland, 2063–2117.
Ellison, Glenn and Edward L. Glaeser. 1997. Geographic concentration in us manufacturing industries: A dartboard approach. Journal of Political Economy 105(5):889–927.
Ellison, Glenn and Edward L. Glaeser. 1999. The geographic concentration of industry: Does natural advantage explain agglomeration? American Economic Review Papers and Proceedings 89(2):311–316.
Ellison, Glenn, Edward L. Glaeser, and William Kerr. 2010. What causes industry agglomeration? Evidence from co-agglomeration patterns. American Economic Review: forthcoming.
Helsley, Robert W. and William C. Strange. 1990. Matching and agglomeration economies in a system of cities. Regional Science and Urban Economics 20(2):189–212.
Henderson, J. Vernon. 1997. Externalities and industrial development. Journal of Urban Economics 42(3):449–470.
Henderson, J. Vernon. 2003. Marshall’s scale economies. Journal of Urban Economics 53(1):1–28.
Holmes, Thomas J. 1999. Localization of industry and vertical disintegration. Review of Economics and Statistics 81(2):314–325.
Krugman, Paul R. 1991. Geography and Trade. Cambridge, ma: mit Press.
Ottaviano, Gianmarco I. P. and Jacques-François Thisse. 2004. Agglomeration and economic geography. In Vernon Henderson and Jacques-François Thisse (eds.) Handbook of Regional and Urban Economics, volume 4. Amsterdam: North-Holland, 2563–2608.
Overman, Henry G. and Diego Puga. 2010. Labour pooling as a source of agglomeration: An empirical investigation. In Edward L. Glaeser (ed.) Agglomeration Economics. Chicago, IL: Chicago University.
Rosenthal, Stuart S. and William Strange. 2004. Evidence on the nature and sources of agglomeration economies. In Vernon Henderson and Jacques-François Thisse (eds.) Handbook of Regional and Urban Economics, volume 4. Amsterdam: North-Holland, 2119– 2171.
Rosenthal, Stuart S. and William C. Strange. 2001. The determinants of agglomeration. Journal of Urban Economics 50(2):191–229.
Rosenthal, Stuart S. and William C. Strange. 2003. Geography, industrial organization, and agglomeration. Review of Economics and Statistics 85(2):377–393.
Scotchmer, Suzanne. 2002. Local public goods and clubs. In Alan J. Auerbach and Martin Feldstein (eds.) Handbook of Public Economics, volume 4. Amsterdam: North-Holland, 1997–2042.
Stigler, George J. 1951. The division of labor is limited by the extent of the market. Journal of Political
Wheaton, William C. and Mark J. Lewis. 2002. Urban wages and labor market agglomeration. Journal of Urban Economics 51(3):542–562.
You might find the Handbook of Regional and Urban Economics by Vernon Henderson and Jacques-François Thisse a particularly useful reference on the sources of agglomeration.