As Ron says, you need to have a look at Philip Guedalla's The Duke which has a long discussion of the books AW took with him to India. This is based on the surviving bill from the bookseller, and it leaves open the imponderable question of how much of it did he ever actually read? Other accounts of long shipboard voyages of the period suggest that the conditions on board were much less conducive to serious study than might have been expected. For example Sir James Mackintosh, who was a serious philosopher and intellectual, made the same trip a few years later (c1804) and read very little, complaining that his books were mostly stored in trunks in the hold and were inaccessible, while the need to socialize with fellow passengers made serious study almost impossible.
I've always had my doubts about the list personally. Many of the items on it have the look of what a good bookseller thought a young gentleman ought to own or have read ie they were 'standard works' of the sort more often purchased than read. For example, Swift's works, in 24 volumes, Adam Smith's Wealth of Nations in 3 vols, Blackstone's Commentaries on the Laws of England in 4 vols, Plutarch's Lives in 6 vols and Locke's Works in 9 vols. That lot alone (and there is much more) would take a good student in favourable conditions more than a year to read, note and digest; and I can't say that one sees much evidence that AW ever tackled much of this - although he probably did read some of the books on India, which figure largely in the list, not only on the voyage, but when he was in India.
For the return voyage he prefered light romantic fiction: but again while this may reflect a change in taste or a change in aspiration it might just as easily be the fruit of experience or what the booksellers in India had available.
The Guedalla is readily available (it has been recently re-done in an inexpensive paperback), and is worth having a look at. It is written with some panache and some people prefer it to Longford; though to my mind it seems that neither Guedalla nor Longford are particularly interested in military campaigns or British politics, and that this is a problem in writing a life of a successful soldier who went on to become prime minister.
From your reference to Le Marchant, I presume you know Thomine's biography of him: Scientific Soldier ?