I don't know if we could call any book on Simon Bolivar the best or even the definitive one. Bolivar is a very contentious figure, because in many ways his shadow stills looms over the Latin American countries he influenced. Moreover, certain relatively recent political movements have appropriated Bolivar's image and name in favor of their ideologies or objectives. That's all I'm going to say due to the 20 year rule, but it's needless to say that it's difficult to separate Bolivar the man and Bolivar the hero and Bolivar the symbol from each other when even nowadays they are all discussed quite contentiously and used to advance political objectives - which does not usually conduct to objective historical analysis.

Even outside of modern politics there's the simple issue that much of what we know of Bolivar comes from biased sources or politically motivated accounts. Bolivar, remember, did not die in calm or glory. He died a pathetic death in Santa Marta, exiled from the countries he had liberated after the complex political process that resulted in the dissolution of Gran Colombia had shattered his legacy and turned several of his friends into bitter enemies. Thus, many of the sources on Bolivar and his life try to paint him as the bloodthirsty tyrant who destroyed Gran Colombia, or the saintly patriot, forsaken by false friends, with precious little middle ground. In that context, can we trust Daniel O'Leary, a friend of Bolivar til the bitter end, who would obviously be interested in defending El Libertador? Can we trust men like Santander and Paez, who had been friends or at least allies of Bolivar but turned into bitter enemies due to their political rivalry?

Even nowadays there are many people still fighting the Santander Bolivar feud, without really reaching a consensus on who was to blame for the fall of Gran Colombia. All this tends to color works on El Libertador and his era - for example, John Lynch's biography of Simon Bolivar is decidedly in favor of him, painting Santander as treacherous and incompetent, while Bolivar's own flaws are minimized. If you read a work with a more positive view of Santander, such as David Bushnell's The Santander Regime in Gran Colombia, Bolivar could be the one that comes across as incompetent and even malicious.

Furthermore, I'd say that a single biography or book would never be enough to understand a topic. If you want to know more about Gran Colombia (sorry, pet peeve of mine), I'd suggest for you to first learn about the wider context of the era before delving into biographies of specific characters. Such overviews are usually less biased, and will allow you to better understand the causes and consequences of the actions of certain historical characters. Focusing on just one man is also never good, in my opinion, because it could make you fall into the trap of Great Man theory - that the actions of a single man are all that direct and shape history. Bolivar, while undoubtedly extremely important, had to contend with other important men and women and with factors outside of his control.

There are some good resources in the independence wars of Hispanic America in English. I'd recommend Chasteen's Americanos, Latin America's struggle for Independence. It's accessible and has a good focus on Bolivar and Gran Colombia (other works, such as the Cambridge History, tend to focus on Mexico), besides including accounts of women and ethnic minorities, who are oftentimes forgotten players. Such inclusiveness is always welcome, and it could help you learn about the simply awesome Manuela Saenz. It's not a comprehensive or exhaustive analysis of the Independence Era, but should be enough to give you a basis from which you can learn more.

Gran Colombia itself is slightly harder. Ecuador, Colombia and Venezuela all downplayed the Gran Colombia era as a mere failed experiment in the way towards becoming truly independent Republics. This was done out of nationalistic reasons, and also because Gran Colombia is Bolivar's bitterest failure, while all countries tried to rehabilitate his memory and rally round him as a symbol of national identity and pride. Thus, scholarship in Gran Colombia is relatively underdeveloped, usually being limited to a brief analysis of how Ecuador, Colombia and Venezuela did while part of Gran Colombia before proceeding with the history of the independent countries. As such, an overview of Gran Colombia, its merits, achievements and failures is hard to come by, most works tinted with the assumption that it would always fail and thus it's not worth it to examine why it failed. The result is that the quite old, but not necessarily outdated, The Santander Regime in Gran Colombia remains the best book on the political and social factors that led Gran Colombia to collapse - this despite Bushnell's disclaim that it isn't the definite work on the country.

So, to summarize, works on Bolivar tend to suffer from the fact that Bolivar was a very contentious figure in his time and remains contentious nowadays, so you must be aware of the biases and even the political ideologies of the authors. I'd recommend, for that reason, to first learn more about the Independence Wars and Gran Colombia from sources that don't focus exclusively on Bolivar, to give you a base that will allow you to understand Bolivar the man and identify possible biases. Lynch's biography, for example, is quite good if you know enough about Santander and the wider sociopolitical context to get a fuller picture of the situation and come to your own conclussions.