The Kingdom of Jerusalem (1952) is volume 2 in the trilogy A History of the Crusades by British historian Steven Runciman. In this excellent work Runciman covers the period from the establishment of the kingdom in 1100 up to its fall at the hands of muslim warrior Saladin in the late 1180s.
During this time, we see a Frankish nobility establishing itself as overlords in the Palestinian and Syrian deserts, never really interested in political stability or the building of a durable empire. Instead we see families vying for power, using marriages to force their way to the throne, making and breaking oaths, assassinations of political rivals, military failures at the hands of incompetent or rash knights, etc. etc.
To be fair, there were a couple of competent rulers, Baldwin I and Baldwin II for example, who brought stability and were able to rise above the pettiness of their peers and look for long term viable geopolitical strategies. But due to genetic weaknesses, a territory full of disease, a harsh climate and endless military conflicts, most rulers saw their lives cut short and the average lifespan for males was drastically lower than their counterparts in Europe. At some points in time there wasn't even a male relative to succeed a deceased one in their position.
Also, after the First Crusade (1095-1100) the willingness for European lords to join in an adventure disappeared, while European monarchs were so busy with fighting their own petty conflicts that they could not and would not spare the men for a holy war. And let's be honest, by now there was nothing holy about this war in the East to begin with. What started as a rallying call against Islam had turned into a pet project of some Frankish nobility with ego's that were too big for their own good.
While all these events took place, the Byzantine Empire slowly collapsed until it lost its entire army in 1176 in a campaign against the Turks, and stopped being a factor of power in the region. At the same time, a lucky and competent Saracen commander, Saladin, achieved the unification of the northern and southern Muslim empires by playing his enemies off against each other and conquering them one by one. The Franks were reduced to Antioch and the petty Muslim states in Syria and Lebanon were all mopped up. With Egypt, Syria, key cities like Damascus and Aleppo, and most of the key transport networks in his possession, he turned his army against Jerusalem.
In short: the Kingdom of Jerusalem was, militarily speaking, continuously understaffed, incompetently ruled and in the midst of geopolitical ruptures beyond their own control. A recipe for disaster.
Runciman tells of all these events and their key players in a very accessible and enjoyable way. Compared to Vol 1, this book is 100 pages longer, while the subject matter really isn't all that different. Of course, there's so much ground to cover that these additional pages can be justified, but the reader ends up with endless accounts of battles, political intrigues, negations, etc. Now, after having read already 650 pages on these subjects, Runciman's style cannot compensate anymore for the growing tediousness and repetition of the subject matter - and there's still 400 pages of Vol 3 to cover...
Also, besides the repetitiveness, Runciman chooses to focus solely on geopolitical, mostly military, developments. Of course this is inevitable when dealing with a time and place in which war, raids and plunder were of existential importance for all parties involved. But one wishes Runciman had offered more insights into the cultures, economic structures, etc. of the region and the era. Unfortunately, in Vol 1 there was no such information while in Vol 2 there's only one chapter dedicated to these subjects.