Curious about the flowers you saw on your last local hike? For starters, pick up a copy of Wildflowers of the San Gabriel Mountains by Ann and Gerald Croissant and, next time you hike in the Angeles National Forest, toss it in your backpack. It's a nice introduction to some of the most common flora. The book is light, small and arranged by blossom color, which is helpful for beginners.
However, be prepared for some frustration: On any single hike you'll only spot a few of these. Still, once you figure out what lupines, salvias, penstemon, etc, look like in general, it will be easier for you to use more detailed field guides. (Here's my list of my faves.) This book also identifies nonnative (invasive) plants that have taken root in the forest, so take note before you tout all the amazing wild mustard plants you've seen.
Although nothing compares to seeing the plants in the wild, one of the easiest ways to learn local flora is to visit Rancho Santa Ana Botanic Garden or stroll a guided (interpretative) trail, such as those near park nature and visitor centers. Because these areas are landscaped, there's usually a higher concentration of flowering plants. (Not to mention, they're labeled!)