I don't know what to suggest right off but what you can do is find a botany course at a local college / university and find out what textbook they use (most have textbooks listed on the schools bookstore's web page)

You can also email the instructor / professor and ask if they recommend that book for a beginner - ask also which book they recommend (sometimes they have no choice but to use the book listed for various reasons) You can see what book(s) they recommend then purchase the book(s) on a site such as amazon.com or another inexpensive used textbook site.

One very inexpensive and concise text is Botany for Gardeners by Brian Capon. This is a text that is used for general horticultural courses so not extremely detailed or technical but certainly addresses all aspects of botany in a basic way. Should provide a helpful overview of the subject to begin with and then you can make some decisions about how to proceed further.

Career options in botany are very broad but tend towards the more technical end of plant studies :-) Have you given any more thought as to what specific type of career in this field you'd like to pursue? Unless you have desires to become a plant scientist, it could be that acquiring a degree in botany is not exactly the best or most efficient/quickest route to take. Many folks who work professionally within the horticultural arena do not have botany degrees but have some other, often less technical plant-related course of study. A visit to your local college or university and a discussion with a guidance counselor may be helpful as well.

Study botany and study internet site building and and maintanance and contribute to a site such as Botany online - The Internet Hypertextbook. One could probably start soon proofreading or translating and learn a lot of botany while working for the site.