Welcome, C.C. You can definitely get good, publishable results with free software. You note "clear & legible" - it's good you know that. SOme first-time authors seem to want to put artistic masterpieces better suiited for four-color lithography at three-feet-by four into standard paperbacks. Not. Going. To. Work. :-) Beyond clarity, can you give us an idea what style you're shooting for? How much detail are you shooting for?

An option might be to put a bare-bones map in the books and link to better stuff online. Genning up an online community around your work seems an effective way to get repeat business and broader notice. 'Course that takes a whole nother skillset and devotion to ongoing effort - might work though. Might also be a place to host tidbits that don't fit in the narrative, yet which seem too good to bury in your notes :-).