Datasheets

AT90USB162 Datasheet (2.3M PDF), for Teensy 1.0

ATMEGA32U4 Datasheet (6.4M PDF), for Teensy 2.0

AT90USB1286 Datasheet (4.7M PDF), for Teensy++ 2.0 (and also Teensy++ 1.0)

MKL26Z64 Manual (5.2M PDF), for Teensy-LC (This manual has all the useful programing info)

MKL26Z64 Datasheet (1.1M PDF), for Teensy-LC (Only the electrical specifications)

Definitive Guide to ARM Cortex-M0 & Cortex-M0+ (book), for Teensy-LC (ARM Processor low-level details)

MK20DX128 Manual (7.2M PDF), for Teensy 3.0 (All the useful peripheral programing info)

MK20DX128 Datasheet (1.8M PDF), for Teensy 3.0 (Only the electrical specifications)

MK20DX256 Manual (8.0M PDF), for Teensy 3.1 & 3.2 (All the useful peripheral programing info)

MK20DX256 Datasheet (1.8M PDF), for Teensy 3.1 & 3.2 (Only the electrical specifications)

MK64FX512 Manual (14.0M PDF), for Teensy 3.5 (All the useful peripheral programing info)

MK64FX512 Datasheet (1.5M PDF), for Teensy 3.5 (Only the electrical specifications)

MK66FX1M0 Manual (19.1M PDF), for Teensy 3.6 (All the useful peripheral programing info)

MK66FX1M0 Datasheet (1.5M PDF), for Teensy 3.6 (Only the electrical specifications)

IMXRT1060 Manual, Rev 3 (28.6M PDF), for Teensy 4.0 & 4.1 (All the useful peripheral programing info)
IMXRT1060 Manual, Rev 2 (27.7M PDF), for Teensy 4.0 & 4.1 (All the useful peripheral programing info)

IMXRT1060 Datasheet (1.9M PDF), for Teensy 4.0 & 4.1 (Only the electrical specifications)

W25Q16JV-DTR Datasheet (2.0M PDF), flash memory on Teensy 4.0

W25Q64JV-DTR Datasheet (3.2M PDF), flash memory on Teensy 4.1

Definitive Guide to ARM Cortex-M3 & Cortex-M4 (book), for Teensy 3.0 - 3.6 (ARM Processor low-level details)

Unfortunately, this information can be difficult to read for non-engineers (and even for engineers too!). Usually this information is only useful for performing direct access to the timers, uart and other on-chip peripherals, when existing example code does not give exactly the results you need.