Texas is ... well, big. This makes the idea of toting a field guide of native and adapted plants for the entire state around on a hike almost comical. Most field guides for Texas are made, actually, for specific plants or specific regions of the state. Luckily, there are several online sites that can help with identifying a plant no matter where in the state of Texas you found it, no matter what kind of plant it is.
Photos and Notes
Take photos of distinguishing features of the plant up-close for detail. Flowers, stem nodes, leaf arrangements, any surface roots, spines, fruits and seeds can all help distinguish one plant from another. In the ubiquitous sandy brush areas of Texas, midday sun might wash out your photos. Photos taken on cloudy days might provide better color and detail. Take notes on where you found the plant, such as near waterways or in sandy soil. If you don't have a camera, note the leaf color, shape, and edge formation, along with bloom details, height, presence of spines, or any other distinguishing feature you notice.
In some areas of the state, collecting samples is not permitted. But if you're in an area where you can do so, be sure to collect a leaf, a flower (if there are any), and a seed still attached to the plant. In some cases, leaves come in bracts. If the bract is small enough, collect it to compare the leaf arrangement. Many (if not most) plants in Texas have some form of spine on them, so (carefully!) collect one of those as well. Jars work best to store samples until you're home, but if you're collecting from several plants, bring along separate plastic bags so you don't jumble your samples.
A number of universities and organizations across Texas have searchable websites with photos to compare your samples or pictures with. Texas Parks and Wildlife offers the Native Plants of Texas Search Engine and the Texas Plant Information Database. The University of Texas at Austin offers the Flora of Texas Database. Texas A&M University maintains several plant identification databases.
Choosing a Field Guide
Field guides are divided not only by region and type of plant (i.e. wildflowers, cactus, trees, so on) but also by the intent of the user. For example, if you're looking for edible native plants, there are several reasons not to choose a generic regional guide. Guides based on edible foods will let you know not only which plants are edible but which parts of the plants and whether it needs to be cooked to avoid it getting you sick. Likewise, guides to poisonous plants, which will let you know specifically what kind of medical treatment you should seek if you encounter them. And Texas is home to a wide variety of poisonous plants.
A Word of Caution
Texas has a wide variety of not only flora but also fauna. While searching for plants, you may run across snakes, toads, scorpions, stinging insects and other potentially dangerous wildlife that call Texas home. Always take care to note your surroundings, especially in the wilder areas far removed from cities. Most creatures would rather leave you alone, but a well-camouflaged rattlesnake sunning itself near that prickly pear cactus you're trying to photograph won't take kindly if you step on it!