I was a late, and I mean LATE, bloomer in high school... in just about every way I can think of, including music.
Although I was SUPER excited about music, I started relatively late compared to my peers, not joining the school jazz band until I was a senior.
Our incredible band director, Mr. Ward, noticed my excitement and suggested I play in as many settings as possible. For the experience, to increase my skillset, etc.
There was a spot in the SIXTH grade concert band for the crash cymbal. You know, the one the monkeys play. And let me remind you I was in TWELFTH grade.
The even funnier thing is virtually every kid (whose *ahem* quarter notes had yet to *drop*) read music better than me.
Anyway, in both concert AND jazz band, I noticed the horn players had music WAY more difficult than my lowly quarter-note walking bass line. And it occurred to me that they'd been reading much more difficult music probably even from earliest lessons as musicians.
Unfortunately this all didn't occur to me until well after even college (lol) but, hey, better late than never! So I began to start practicing the things horn players have to read.
One nice thing about trombone music is it's already in bass clef with no transposition required. And here is, hands down, what I feel is the absolute best bass book for sight reading: Arban's Famous Method for Trombone, Platinum Edition. [Edit: I'm only seeing used copies of the platinum edition on Amazon, so this is the link for the regular version.]
Or if you want to throw some pennies my way with the affiliate link, you can use this link.
Either way, grab the book, you won't be sorry! And I might also suggest one important guideline when reading:
***If you at any point you notice yourself "falling off the page" so to speak, slow the tempo wayyyy down. Accuracy is more important than speed... Speed will come over time and should not be a concern in the practice room. Seriously, 40 bpm where the 40 equals a SIXTEENTH note is quite alright! As the legendary bass teacher Whit Browne always repeats, "The slow way is the fast way." Surprisingly, you'll improve faster and increase your speed faster when you slow down the tempo to a speed that's readable for you. It's counter-intuitive, but it's true!