You should use the dye fixer to ensure optimum color and water fastness. Heaven forbid the book got wet, but if it did, the surface dyestuff would come off or precipitate to other areas of the leather where you may not want it, and worst-case, into the pages.
All dyes, no matter the dyed material, require a fixative for longevity and fastness, especially on leather before finishing with something water based. Some dyes out there are substantive, meaning that they contain an amount of fixing agent, but they still fade and tend to not be water fast unless finished. In that case, the finish or glaze itself can rub off surface dye, which is the worst thing you'd want to happen when you're so close to the finish line. Definitely use fixer. You don't want any potential clients coming back to you and giving you a what-for about dye fading.
I can tell you from experience that using the dye manufacturer's fixer is your best option. It's specially formulated and tested against their own dyes. Local alternatives may not be what you need and would probably not change the molecular bonds enough to fully fix the dye, seeing as how no two manufacturers use the same formulas. To my knowledge I don't know of any universal alternative to all the synthetic dyes available... but my knowledge might be out of date. I've abandoned aniline dyes and use only natural now.