Barnes & Noble is to stop making its Nook tablets, in a move which could hasten the break-up of the company. The move partly reflects the disappointing results of its relationship with Microsoft so far. When the Windows giant invested in the bookseller’s digital unit last year, it was expected to result in the Nook platform being converted to Windows and becoming part of Microsoft’s device strategy. Instead, that strategy was centered on the homegrown Surface range, and B&N was left with an attractive product which, nevertheless, lacked the scale and differentiation to take on the big names in the crowded Android tablet space. However, relieved of tablet manufacturing costs and the challenges of competing head-on with the iPad, Nook…