Regrettably there is a lack of information about the Hospitallers' medical activities in the Holy Land, simply because contemporaries preferred to write about their military activities.
There are two surviving manuscripts that describe the Hospitallers’ medical care in Jerusalem.
(1) Vatican Lat. 4852, fols 89r-105r:
Transcription and study: Katja Klement, ‘“Von Krankenspeisen und Aurzten ...” Eine unbekannte Verfuugung des Johannitermeisters Roger des Moulins (1177-1187) im Codex Vaticanus Latinus 4852’, unpublished Ph.D. thesis, University of Salzburg, June 1996;
Study: Katja Klement, ‘Alcune osservazioni sul Vat. Lat. 4852’, in Studi Melitensi, 3 (1995);
Study by Susan Edgington, ‘Medical Care in the Hospital of St John in Jerusalem’, in The Military Orders, vol. 2: Welfare and Warfare, ed. Helen Nicholson (Aldershot: Ashgate, 1998);
Edition and translation by Susan B Edgington, ‘Administrative regulations for the hospital of St John in Jerusalem dating from the 1180s’ (includes an edition and English translation of a set of Old French regulationsfor the Hospitallers in Jerusalem from MS. Città del Vaticano, B.A.V., Vat. lat. 4852), Crusades, 4 (2005), 21-37. ISSN: 1476-5276.
(2) Munich Clm. 4620, fols 132v-139v:
Published and discussed by Benjamin Z. Kedar, ‘A Twelfth-Century Description of the Jerusalem Hospital’, in The Military Orders, vol. 2: Welfare and Warfare, ed. Helen Nicholson (Aldershot: Ashgate, 1998), pp 3-26;
Also published and discussed by: Alain Beltjens, Le récit d'une journée au grand hôpital de Saint-Jean de Jérusalem sous le règne des derniers rois latins ayant résidé à Jérusalem (Paris, 2004).
There are also contemporary descriptions of the Hospitallers' medical work in:
John of Würzburg, trans. Aubrey Stewart, in Palestine Pilgrims Text Society, vol. 5 (London, 1891), chapters 5 and 15, also published in Jerusalem Pilgrimage 1099-1185, ed. J. Wilkinson, J. Hill and W.F. Ryan, Hakluyt Society, 2nd series vol. 167 (1988);
Theodoric: Theoderich’s description of the Holy Places, circa 1172 AD, trans. Aubrey Stewart, Palestine Pilgrims Text Society, vol. 5 (London, 1891), also published in Jerusalem Pilgrimage.
Another useful secondary source is:
Malcolm Barber, ‘The Charitable and Medical Activities of the Hospitallers and Templars’, in A History of Pastoral Care, ed. Gillian R. Evans (London, 2000), pp. 149-68.
There is also a fictional source (c. 1260) which depicted Saladin staying at the Hospital at Acre: Récits d`un ménestrel de Reims au treizième siècle, ed. N. de Wailly, Société de l'Histoire de France (Paris, 1876), pp. 104-9, 112, paras 198-208, 213. There is a new edition: Marie-Geneviève Grossel, Le Ménestrel de Reims (Valenciennes, 2002); there is also an English translation in E. N. Stone, Three Old French Chronicles of the Crusades (Seattle, 1939).