Complex sentence: He soon published his "Campagne de 1815", in the preparation of which he had had some help from Napoleon; but Gourgaud's "Journal de Ste-Hélène" was not published till the year 1899.
Split into simpler sentences: He soon published his "Campagne de 1815", in the preparation of which he had had some help from Napoleon. However, Gourgaud's "Journal de Ste-Hélène" was not published till the year 1899.