Maps: Compared to a lot of major world cities, Beijing seems hopelessly never able to catch up to reality on the mapping issue, Chinese or English. Most maps of Beijing are either somewhat or completely out of date on the details and even some of the critical new major roads. So, I would just pick the latest one I could find to get familiar with the general layout of the city and where the major landmark sights are. At least the locations of those haven't changed! One useful thing to have on that map are the latest subway lines and stations--some maps may have these showing as "proposed." When you get to Beijing, make it one of your first missions to see if you can find something better but don't obsess about this.

Guidebooks: If you have the time, try the cheap method and visit the local library. Focus on the Beijing sightseeing parts not the accom/food/misc stuff. Some guidebooks have more historical or cultural background than others but it's a matter of personal preference for how deeply you get into this. Buy your own copy of the one that seems most useful to you. If you are a speed reader/skimmer and your library isn't well stocked, then visit the local bookstore instead and pick one. Some are drier and fact-based, others give more explicit or implicit opinion about the sights. I don't think there is an overall "best guidebook" that you'd get any consensus on. Thrifty option or if you can't make up your mind: go to a secondhand/used/remainder bookstore and buy two or more different guidebooks, then slice out the relevant Beijing parts to ease the weight. For the major sights, it doesn't matter too much if the book is a few years old--the hours of operation will be similar though the admission price may be up. Some of the newer museums (Capital, Beijing Museum of City Planning) may not be in a guidebook unless quite recent, but after some recent Beijing forum research (Tripadvisor, Lonely Planet, Frommers, Bootsnall, etc), you can add things of interest to material you already have.

Oddball: Around bookstores and expat grocery stores in Beijing there is a little deck-of-cards collection in a small box called "Beijing by Foot." It's set up for walkers as a set of card-sized maps with routes and places of interest plotted on them and a bit of description on the reverse side. As it is fairly recent, it is as accurate as any of the larger foldout maps and having tried some of their routes, I can attest to this. You don't get the "big view" of Beijing but as a large map supplement, especially for anything inside the 2nd Ring Road, it is useful and easy to carry/refer to the cards while wandering around. It's priced about the same as a standard map but not sure if available outside Beijing. It's published by the same people as the Insiders Guide to Beijing--which BTW I don't advocate for tourists--it's too heavy and has too much info useful only to expats so is the wrong tool.