As far as geography (which is never a reliable metric in such matters, but does shed some light), Argos was from the beginning a powerful polis. The Argolid (that is, the chora or territory attached to the astu urban area itself) was quite large by the standards of most Greek cities, with a well defined territory which was more or less agriculturally productive and a major shrine to Hera at the Heraion which from very early on was an important religious site both within Argive society and within Greek society at large. The Argive plain was populous, with many sub-city population centers, some of former prestige, like Mycenae and Tiryns. The agricultural might of the city was evident early on, and agricultural territory in Archaic Greek society lends itself to larger hoplite forces. The name "Argos" might be related to the IE root ag/agr "field, farm" (cf Latin ager 'field'). Argos, like Athens and Corinth, had undergone extensive synoikismos, or the process by which early, smaller poleis were absorbed into a larger polis entity. This contributed to the size, population, and prosperity of the city by the 6th century. Though not a coastal city herself, Argos had sheltered access to the sea and a major port at Nauplia, which was another smaller city-state which had been absorbed early on. The Argolid was hemmed in by mountains on west, north, and east, and open to the sea on the south, an ideal situation for delimiting and maintaining borders.

Politically, Archaic Argos was always involved in a complicated dance with Sparta, Corinth, and to a lesser extent Megara. By the time of the late 6th century, when the Persian empire had annexed the Ionian Greek cities and was beginning to exert its influence on the eastern Med., these three (Argos, Sparta, and Corinth) were the main players in the Peloponnese, with Athens and Thebes bringing up the balance. There were many other powerful cities at the time, but these were the main players of the mainland. Sparta drew her power from her military domination of her neighbors, from which she maintained a permanent subservient population (the helots). Sparta's history within the Greek world is very much aberrant, and her power relative to the other cities is very much the result of her odd structure and evolution. Sparta aside, Argos was one of the most powerful states in the Greek world leading up to the events of the Persian Wars.

In the 5th century, as a result of the encroachment of Persian power, we see an evolution of state formation in the Greek world with the increased emphasis on multi-state alliances and super-political hierarchies. They were nothing new, but in the face of Persian dominance, they became the norm for most of the Greek world. Athens had her famous "Delian League" and the resultant thalassocracy. But Sparta also had a confederation, older than the Delian League, called the Peloponnesian League. The politics of this League and its members impacts Argos greatly in the 5th century. Corinth in particular had a secure spot in the league and a clear role, being by far the most powerful naval asset to the Pelop. League and in addition the member state which controlled all land access to the Peloponnese via the Isthmus. The Spartans increasingly depended on Corinth to maintain the threat of land-based invasion of Attica. Corinth continually leveraged her privileged place in the League at the expense of Argos, who variously tried to play nice with the League or go her own way. Unfortunately, by the middle of the 5th century there was little room to maneuver for a powerful but "independent" Greek polis not aligned with Sparta or Athens.

I won't do a mini history of Argive politics here, but I will say that it is not straightforward to compare the "power" of Greek cities. In 491, on the eve of the battle of Marathon, Argos would have considered herself one of the foremost cities in Greece: rich in agriculture and horses, with a long, important tradition and important Greek cultural sites within her borders. She compared favorably to the other hoplite armies of the Greeks, save for the Lakedaimonians, and she had a powerful economy. Sparta, on the other hand, was a fearsome military threat on land but was also handicapped by the very domestic situation which enabled her power. Corinth also had a powerful economy, sufficient hoplites, and a powerful fleet. Athens, in 491, was large in land and economy, about average in hoplites for her size, but yet to "awaken" her power (thanks in part to a subsequent discovery of a motherlode of silver in her southern territory at Laurion, which the Athenians famously decided to use to bolster their naval assets).

I don't have any comment about Argos being an "unreliable" ally. Who said it? Where? When? But you must understand that both Corinth, Argos, and the other Peloponnesian cities were constantly faced with the threat of hoplite warfare/invasion both with each other and with the Spartans. Spartan warfare, or the threat of it, dominates the history of the Peloponnese in the Archaic period. Both Corinth and Argos were much too powerful to be subjugated outright by Sparta like her other victims, especially by the sixth century, but the implicit threat of Lakedaimonian violence was ever-present.

To add to Alkibiades post above, you could also consider Argos’ foreign policy decisions to explain their survival among the great powers. As Alcibiades explained Argos was in its own right a powerful city state, on its own it could not stand up to the Peloponnesian League but when opportunities arose they challenged Sparta for dominance in the 5th century BC. The biggest setback for Argos however was in the mid 490s BC when Argos lost to Sparta under Cleomenes in the Battle of Sepeia. Herodotus claims the Argives suffered 6,000 casualties - though the number might be exaggerated the suggested loss of soldiers would be enough to completely stunt Argive military power for the next generation and would assert Spartan hegemony in the Peloponnese.

I would then characterise Argive geopolitical strategy in the rest of the 5th century as being one of opportunism to regain a foothold in the Peloponnese. This allowed them to survive the tumultuous period relatively unscathed. Then when opportunities arose they would push to regain power. Some examples:

• During Xerxes’ invasion they remained neutral, citing shared kinship with the Persians through Perseus - though Herodotus rightly points out the more probably cause was disdain for Spartan leadership over the Greek resistance forces. By remaining neutral and not medising as other states such as Thebes had done Argos was able to avoid military commitments for either side and emerging unscathed no matter the outcome.
• When the Hellenic league broke down following the Spartan earthquake in 464 BC and the helot revolt that followed, Argos immediately sided with the disgruntled Athens. This gave Argos a powerful ally in any upcoming struggle with Sparta, which did occur over the two following decades as part of the wider conflict called the First Peloponnesian War.
• At the end of that conflict Sparta and Athens signed a peace treaty called the Thirty Years Peace. One of the explicit terms of the treaty was that no alliance was to exist between Athens and Argos - clearly indicating Spartan concerns over these two states allying together to undermine Spartan hegemony. This demonstrates how Argos could still challenge Spartan dominance despite being now militarily and geopolitically weaker - by allying with powerful friends.
• In 421 BC when a temporary armistice was signed between Athens and Sparta during the Peloponnesian War, many Peloponnesian states such as Corinth were disgruntled at Sparta’s agreement to the treaty with Athens and according to Thucydides sought secret alliance with none other than Argos. This panicked Sparta so much that she temporarily sought Athens as a potentially ally despite fighting a bloody war against them over the last ten years! So another method of Argive power was to attempt to undermine the Peloponnesian alliance when opportunities arose.
• This also materialised further in 418 BC when Argos, at the advice of Alcibiades (not OP but the real Alcibiades!) signed an alliance that included Athens, Argos, Elis, Mantinea and many Arcadian allies. Sparta was so concerned about this that she mustered the Peloponnesian forces and marched into the Argolid under King Agis II. According to Thucydides Agis and the Argives then hastily concluded a treaty, that was so disliked that when Agis arrived back in Sparta he was fined by the Ephors and they threatened to tear his house down in punishment. Meanwhile Argos and Athens mustered their forces and a battle was fought between Sparta and the quadruple alliance (Argos, Athens, Mantinea and Elis) at the battle of Mantinea in 418 BC - the largest hoplites battle of the Peloponnesian War. It was a Spartan victory but they could not follow it up with a counterattack against Argos - thus enabling Argos to recover but signing a peace treaty with Sparta.
• After 418 they largely remained neutral in the Peloponnesian War again - avoiding again threatening their own position by siding with the losing side.

So to summarise Argos maintained its power by opportunism- they would remain neutral when circumstances demanded, and then form careful alliances with other powerful states when opportunities arose to undermine Spartan control in the Peloponnese - such as Athens and Corinth.

I noticed you asked about possible books - I’d recommend Thucydides History of the Peloponnesian War if you’re interested in Greek geopolitics in the 5th century BC. It’s not an easy read but there’s no book better for the period. A modern book I would recommend would be Terry Buckley’s “Aspects of Greek History” as he focuses on the primary sources when constructing his narrative.