Unsurpassable as a history of the crusades. Sir Steven Runciman, possibly one of the greatest historians we have ever seen, learnt to read a huge number of languages - not just Greek plus Latin and its later derivatives, but also Arabic, Syriac, Hebrew, Turkish, Persian, Bulgarian, Russian, Armenian, Georgian and probably others too - we may never see his like again. This wide ranging linguistic skill enabled him to go straight to the original sources in his research. For this reason, it's unfair and totally unwarranted, as at least one reviewer here has done, to accuse Runciman of displaying a biased viewpoint from the Christian side only.
In the modern age, political correctness demands that the Christian side must always be seen as murderous bloodthirsty savages to a man whilst the Muslims must be seen as invariably cultured and peaceful and unable to do wrong. In a pre-PC age Runciman is quite objective in his treatment of all sides - no-one was incapable of great atrocities, it was the nature of the age, even if the Franks were worse. His sympathies however, as a Byzantinist and admirer of Byzantine culture, do lean towards the Byzantines; he sees the era as being in effect the last act in the long history of barbarian invasions of the Roman empire - the uncouth destructive barbarian Frankish hordes overrunning the cultured Byzantium and the Middle East, the nadir being the Crusaders' capture of Constantinople in 1204.
Certainly it's hard going, but such a thoroughly researched work covers the full complexities of the history of a period often envisaged simplistically in the popular imagination as one of Christian versus Muslim, but which is more correctly one of a constantly shifting power game of rivalries and alliances - by turns Frank against Frank, Frank against Norman, Norman against Byzantine, Frank against Byzantine, Byzantine against Turk, Turk against Arab, Arab against Arab, Sunni against Shia, Catholic against Orthodox and so on.