The influence of skeptical thinkers on Shelley's poetry and philosophy was decisive. There is unfortunately very little writing on the subject which is approachable and easy to digest.  There are several important books which cover the subject. Two are focused exclusively on the question:

Pulos, C. E. (1954) The Deep Truth: A Study of Shelley's Scepticism, Lincoln, NE: University of Nebraska.

Hoagwood, Terence Allan (1989) Scepticism and Ideology: Shelley's Political Prose and its Philosophical Context from Bacon to Marx, Iowa City, IA: University of Iowa Press.

Some touch on the subject in depth:

Wasserman, Earl R. (1971) Shelley: A Critical Reading, Baltimore and London: Johns Hopkins Press.

Duffy, Cian (2009) Shelley and the Revolutionary Sublime, Cambridge: Cambridge University Press.

All of these are what I would consider to be specialist texts which require an enormous amount of effort to digest.  Thomas Holden's article therefore fills an important gap by canvassing the question in a more digestible fashion.  The great service which Holden offers is that he zeros in on Sir William Drummond himself - a key figure in the philosophical landscape of the last 18th and early 19th centuries, and a thinker who who exerted an enormous influence on Shelley.  Shelley actually met him in Italy.

It is a unfortunate that Drummond's book, "Academical Questions" is almost completely unavailable to the general reading public. A recent reissue has disappeared into thin air.  Until it reappears, we have Professor Holden!