Well there isn't an easy answer, because the answer is different for everyone.
First, look at your data! Do a stakeholder survey of topics that are most relevant to them. I use Google Drive to create an easy Google Form that I can email to parents and teachers. I can also put it online for students. Also, check out your referrals. What are you getting the most referrals for? Next, check your current inventory, do you have books on those topics? If not, start there! If bullying is not a top issue in your school, but study skills are then you know what books will be getting the most use. So as much as I love the Weird Series for bullying, I would start putting my funds towards books like Annie's Plan.
What is the focus of your school? Where are you heading? I have tons of books on friendship, but I noticed this year I needed more books on handling stress and more titles geared towards our PBIS focus.
Look at your scope and sequence. When I plan out my year of classroom guidance lessons I always partner it with a book. This helps me gauge what topics I might need, but also helps me focus on what books I really want to use.
What do others love? Some titles are must haves like Mr. Peabody's Apples or What If Everybody Did That?. Check out what other counselors are using all the time.
Here are some of my favorite titles:
Here are some authors you can't go wrong with:
Corey Rosen Schwartz
Need more ideas? Check out my Good Reads page here: https://www.goodreads.com/PawsitiveSchoolCounseling